feat: add chat session history with IndexedDB persistence (#500)

* feat(session): add chat session history with IndexedDB storage

- Add session-storage.ts with IndexedDB wrapper using idb library
- Add use-session-manager.ts hook for session state management
- Add session-history-dropdown.tsx for session selection UI
- Integrate session system into chat-panel.tsx
- Auto-generate session titles from first user message
- Auto-save sessions on message completion
- Support session switching, deletion, and creation
- Migrate existing localStorage data to IndexedDB
- Add i18n translations for session history UI

* feat(session): improve history dropdown and persist diagram history

- Add time-based grouping (Today, Yesterday, This Week, Earlier)
- Add thumbnail previews using Next.js Image component
- Add staggered entrance animations with fade-in effects
- Improve active session indicator with left border accent
- Fix scrolling by using native overflow instead of ScrollArea
- Persist diagram version history to IndexedDB sessions
- Remove redundant diagram XML from localStorage
- Add i18n strings for time group labels (en, ja, zh)

* fix(session): prevent data loss on theme change and tab close

- Add isDrawioReady effect to restore diagram after DrawIO remount
- Add visibilitychange handler to save session when page becomes hidden
- Fix missing currentSessionId in saveCurrentSession dependency array
- Remove unused sanitizeMessages import from use-session-manager

* fix(session): fix diagram save and migration data loss bugs

- Add diagramHistory to save effect dependency array so diagram-only
  edits trigger saves (previously only message changes did)
- Destructure stable sessionManager values to prevent unnecessary
  effect re-runs on every render
- Add try-catch wrapper around debounced async save operation
- Make saveSession() return boolean to indicate success/failure
- Verify IndexedDB write succeeded before deleting localStorage data
  during migration (prevents data loss if write silently fails)
- Keep localStorage data for retry if migration fails instead of
  marking as complete anyway

* refactor(session): extract helpers to reduce code duplication

- Add syncUIWithSession helper to consolidate 4 duplicate UI sync blocks
- Add buildSessionData helper to consolidate 4 duplicate save logic blocks
- Remove unused saveTimeoutRef and its cleanup effect
- Net reduction of ~80 lines of duplicate code

* style(ui): improve history dropdown and delete dialog styling

- Change destructive color from coral to muted rose for refined look
- Make session history panel taller (400px fixed height)
- Fix popover alignment to prevent truncation
- Style delete button with soft red outline instead of solid fill
- Make delete dialog more compact (max-w-sm)

* fix(session): reset refs on new chat and show recent sessions

- Fix cached example diagrams not displaying after creating new session
- Reset previousXML, lastProcessedXmlRef and processedToolCalls when
  messages become empty (new chat or session switch)
- Add recent chats section in empty chat state with collapsible examples
- Pass sessions and onSelectSession to ChatMessageDisplay
- Add loadedMessageIdsRef to skip animations on session restore
- Add debug console.log for diagram processing flow

* feat(session): add search bar and improve history UI

- Remove session history dropdown, use main panel instead
- Add search bar to filter history chats by title
- Show minutes (Xm ago) instead of "Just now" for recent sessions
- Scroll to top when switching to new/empty chat
- Remove title truncation limit for better searchability
- Remove debug console.log statements

* refactor: remove redundant code and fix nested button hydration error

- Remove unused 'sessions' from deleteSession dependency array
- Remove unused 'switchedTo' variable and simplify return type
- Remove unused 'restoredMessageIdsRef' (always empty)
- Fix nested button hydration error by using div with role=button
- Simplify handleDeleteSession callback

* fix(session): fix migration bug, improve metadata perf, truncate titles

- Fix migration retry loop when localStorage has empty array
- Use cursor-based iteration for getAllSessionMetadata
- Truncate session titles to 100 chars with ellipsis

* refactor: remove dead code and extract diagram length constant

- Remove unused exports: getAllSessions, createNewSession, updateSessionTitle
- Remove write-only CURRENT_SESSION_KEY and all localStorage calls
- Remove dead messagesEndRef and unused scroll effect
- Extract magic number 300 to MIN_REAL_DIAGRAM_LENGTH constant
- Add isRealDiagram() helper function for semantic clarity

contexts/diagram-context.tsx

@@ -3,15 +3,19 @@
 import type React from "react"
 import { createContext, useContext, useEffect, useRef, useState } from "react"
 import type { DrawIoEmbedRef } from "react-drawio"
-import { STORAGE_DIAGRAM_XML_KEY } from "@/components/chat-panel"
 import type { ExportFormat } from "@/components/save-dialog"
 import { getApiEndpoint } from "@/lib/base-path"
-import { extractDiagramXML, validateAndFixXml } from "../lib/utils"
+import {
+    extractDiagramXML,
+    isRealDiagram,
+    validateAndFixXml,
+} from "../lib/utils"
 
 interface DiagramContextType {
     chartXML: string
     latestSvg: string
     diagramHistory: { svg: string; xml: string }[]
+    setDiagramHistory: (history: { svg: string; xml: string }[]) => void
     loadDiagram: (chart: string, skipValidation?: boolean) => string | null
     handleExport: () => void
     handleExportWithoutHistory: () => void
@@ -24,7 +28,7 @@ interface DiagramContextType {
         format: ExportFormat,
         sessionId?: string,
     ) => void
-    saveDiagramToStorage: () => Promise<void>
+    getThumbnailSvg: () => Promise<string | null>
     isDrawioReady: boolean
     onDrawioLoad: () => void
     resetDrawioReady: () => void
@@ -41,72 +45,52 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
         { svg: string; xml: string }[]
     >([])
     const [isDrawioReady, setIsDrawioReady] = useState(false)
-    const [canSaveDiagram, setCanSaveDiagram] = useState(false)
     const [showSaveDialog, setShowSaveDialog] = useState(false)
     const hasCalledOnLoadRef = useRef(false)
     const drawioRef = useRef<DrawIoEmbedRef | null>(null)
     const resolverRef = useRef<((value: string) => void) | null>(null)
     // Track if we're expecting an export for history (user-initiated)
     const expectHistoryExportRef = useRef<boolean>(false)
-    // Track if diagram has been restored from localStorage
+    // Track if diagram has been restored after DrawIO remount (e.g., theme change)
     const hasDiagramRestoredRef = useRef<boolean>(false)
+    // Track latest chartXML for restoration after remount
+    const chartXMLRef = useRef<string>("")
 
     const onDrawioLoad = () => {
         // Only set ready state once to prevent infinite loops
         if (hasCalledOnLoadRef.current) return
         hasCalledOnLoadRef.current = true
-        // console.log("[DiagramContext] DrawIO loaded, setting ready state")
         setIsDrawioReady(true)
     }
 
     const resetDrawioReady = () => {
-        // console.log("[DiagramContext] Resetting DrawIO ready state")
         hasCalledOnLoadRef.current = false
         setIsDrawioReady(false)
     }
 
-    // Restore diagram XML when DrawIO becomes ready
-    // eslint-disable-next-line react-hooks/exhaustive-deps -- loadDiagram uses refs internally and is stable
+    // Keep chartXMLRef in sync with state for restoration after remount
     useEffect(() => {
-        // Reset restore flag when DrawIO is not ready (e.g., theme/UI change remounts it)
+        chartXMLRef.current = chartXML
+    }, [chartXML])
+
+    // Restore diagram when DrawIO becomes ready after remount (e.g., theme/UI change)
+    useEffect(() => {
+        // Reset restore flag when DrawIO is not ready (preparing for next restore cycle)
         if (!isDrawioReady) {
             hasDiagramRestoredRef.current = false
-            setCanSaveDiagram(false)
             return
         }
+        // Only restore once per ready cycle
         if (hasDiagramRestoredRef.current) return
         hasDiagramRestoredRef.current = true
 
-        try {
-            const savedDiagramXml = localStorage.getItem(
-                STORAGE_DIAGRAM_XML_KEY,
-            )
-            if (savedDiagramXml) {
-                // Skip validation for trusted saved diagrams
-                loadDiagram(savedDiagramXml, true)
-            }
-        } catch (error) {
-            console.error("Failed to restore diagram from localStorage:", error)
+        // Restore diagram from ref if we have one
+        const xmlToRestore = chartXMLRef.current
+        if (isRealDiagram(xmlToRestore) && drawioRef.current) {
+            drawioRef.current.load({ xml: xmlToRestore })
         }
-
-        // Allow saving after restore is complete
-        setTimeout(() => {
-            setCanSaveDiagram(true)
-        }, 500)
     }, [isDrawioReady])
 
-    // Save diagram XML to localStorage whenever it changes (debounced)
-    useEffect(() => {
-        if (!canSaveDiagram) return
-        if (!chartXML || chartXML.length <= 300) return
-
-        const timeoutId = setTimeout(() => {
-            localStorage.setItem(STORAGE_DIAGRAM_XML_KEY, chartXML)
-        }, 1000)
-
-        return () => clearTimeout(timeoutId)
-    }, [chartXML, canSaveDiagram])
-
     // Track if we're expecting an export for file save (stores raw export data)
     const saveResolverRef = useRef<{
         resolver: ((data: string) => void) | null
@@ -132,27 +116,32 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
         }
     }
 
-    // Save current diagram to localStorage (used before theme/UI changes)
-    const saveDiagramToStorage = async (): Promise<void> => {
-        if (!drawioRef.current) return
+    // Get current diagram as SVG for thumbnail (used by session storage)
+    const getThumbnailSvg = async (): Promise<string | null> => {
+        if (!drawioRef.current) return null
+        // Don't export if diagram is empty
+        if (!isRealDiagram(chartXML)) return null
 
         try {
-            const currentXml = await Promise.race([
+            const svgData = await Promise.race([
                 new Promise<string>((resolve) => {
                     resolverRef.current = resolve
                     drawioRef.current?.exportDiagram({ format: "xmlsvg" })
                 }),
                 new Promise<string>((_, reject) =>
-                    setTimeout(() => reject(new Error("Export timeout")), 2000),
+                    setTimeout(() => reject(new Error("Export timeout")), 3000),
                 ),
             ])
 
-            // Only save if diagram has meaningful content (not empty template)
-            if (currentXml && currentXml.length > 300) {
-                localStorage.setItem(STORAGE_DIAGRAM_XML_KEY, currentXml)
+            // Update latestSvg so it's available for future saves
+            if (svgData?.includes("<svg")) {
+                setLatestSvg(svgData)
+                return svgData
             }
-        } catch (error) {
-            console.error("Failed to save diagram to storage:", error)
+            return null
+        } catch {
+            // Timeout is expected occasionally - don't log as error
+            return null
         }
     }
 
@@ -273,9 +262,6 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                     fileContent = xmlContent
                     mimeType = "application/xml"
                     extension = ".drawio"
-
-                    // Save to localStorage when user manually saves
-                    localStorage.setItem(STORAGE_DIAGRAM_XML_KEY, xmlContent)
                 } else if (format === "png") {
                     // PNG data comes as base64 data URL
                     fileContent = exportData
@@ -346,6 +332,7 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                 chartXML,
                 latestSvg,
                 diagramHistory,
+                setDiagramHistory,
                 loadDiagram,
                 handleExport,
                 handleExportWithoutHistory,
@@ -354,7 +341,7 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                 handleDiagramExport,
                 clearDiagram,
                 saveDiagramToFile,
-                saveDiagramToStorage,
+                getThumbnailSvg,
                 isDrawioReady,
                 onDrawioLoad,
                 resetDrawioReady,