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

app/[lang]/page.tsx

@@ -1,6 +1,6 @@
 "use client"
 import { usePathname, useRouter } from "next/navigation"
-import { useCallback, useEffect, useRef, useState } from "react"
+import { Suspense, useCallback, useEffect, useRef, useState } from "react"
 import { DrawIoEmbed } from "react-drawio"
 import type { ImperativePanelHandle } from "react-resizable-panels"
 import ChatPanel from "@/components/chat-panel"
@@ -22,7 +22,6 @@ export default function Home() {
         handleDiagramExport,
         onDrawioLoad,
         resetDrawioReady,
-        saveDiagramToStorage,
         showSaveDialog,
         setShowSaveDialog,
     } = useDiagram()
@@ -110,8 +109,7 @@ export default function Home() {
         onDrawioLoad()
     }, [onDrawioLoad])
 
-    const handleDarkModeChange = async () => {
-        await saveDiagramToStorage()
+    const handleDarkModeChange = () => {
         const newValue = !darkMode
         setDarkMode(newValue)
         localStorage.setItem("next-ai-draw-io-dark-mode", String(newValue))
@@ -120,8 +118,7 @@ export default function Home() {
         resetDrawioReady()
     }
 
-    const handleDrawioUiChange = async () => {
-        await saveDiagramToStorage()
+    const handleDrawioUiChange = () => {
         const newUi = drawioUi === "min" ? "sketch" : "min"
         localStorage.setItem("drawio-theme", newUi)
         setDrawioUi(newUi)
@@ -129,7 +126,7 @@ export default function Home() {
         resetDrawioReady()
     }
 
-    // Check mobile - save diagram and reset draw.io before crossing breakpoint
+    // Check mobile - reset draw.io before crossing breakpoint
     const isInitialRenderRef = useRef(true)
     useEffect(() => {
         const checkMobile = () => {
@@ -138,7 +135,6 @@ export default function Home() {
                 !isInitialRenderRef.current &&
                 newIsMobile !== isMobileRef.current
             ) {
-                saveDiagramToStorage().catch(() => {})
                 setIsDrawioReady(false)
                 resetDrawioReady()
             }
@@ -150,7 +146,7 @@ export default function Home() {
         checkMobile()
         window.addEventListener("resize", checkMobile)
         return () => window.removeEventListener("resize", checkMobile)
-    }, [saveDiagramToStorage, resetDrawioReady])
+    }, [resetDrawioReady])
 
     const toggleChatPanel = () => {
         const panel = chatPanelRef.current
@@ -266,16 +262,24 @@ export default function Home() {
                     onExpand={() => setIsChatVisible(true)}
                 >
                     <div className={`h-full ${isMobile ? "p-1" : "py-2 pr-2"}`}>
-                        <ChatPanel
-                            isVisible={isChatVisible}
-                            onToggleVisibility={toggleChatPanel}
-                            drawioUi={drawioUi}
-                            onToggleDrawioUi={handleDrawioUiChange}
-                            darkMode={darkMode}
-                            onToggleDarkMode={handleDarkModeChange}
-                            isMobile={isMobile}
-                            onCloseProtectionChange={setCloseProtection}
-                        />
+                        <Suspense
+                            fallback={
+                                <div className="h-full bg-card rounded-xl border border-border/30 flex items-center justify-center text-muted-foreground">
+                                    Loading chat...
+                                </div>
+                            }
+                        >
+                            <ChatPanel
+                                isVisible={isChatVisible}
+                                onToggleVisibility={toggleChatPanel}
+                                drawioUi={drawioUi}
+                                onToggleDrawioUi={handleDrawioUiChange}
+                                darkMode={darkMode}
+                                onToggleDarkMode={handleDarkModeChange}
+                                isMobile={isMobile}
+                                onCloseProtectionChange={setCloseProtection}
+                            />
+                        </Suspense>
                     </div>
                 </ResizablePanel>
             </ResizablePanelGroup>

app/globals.css

@@ -74,8 +74,8 @@
     --accent: oklch(0.94 0.03 280);
     --accent-foreground: oklch(0.35 0.08 270);
 
-    /* Coral destructive */
-    --destructive: oklch(0.6 0.2 25);
+    /* Muted rose destructive */
+    --destructive: oklch(0.45 0.12 10);
 
     /* Subtle borders */
     --border: oklch(0.92 0.01 260);
@@ -122,7 +122,7 @@
     --accent: oklch(0.3 0.04 280);
     --accent-foreground: oklch(0.9 0.03 270);
 
-    --destructive: oklch(0.65 0.22 25);
+    --destructive: oklch(0.55 0.12 10);
 
     --border: oklch(0.28 0.015 260);
     --input: oklch(0.25 0.015 260);