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

lib/i18n/dictionaries/en.json

@@ -212,6 +212,22 @@
         "contactMe": "Contact Me",
         "usageNotice": "Due to high usage, I have changed the model from Claude to minimax-m2 and added some usage limits. See About page for details."
     },
+    "sessionHistory": {
+        "tooltip": "Chat History",
+        "newChat": "New Chat",
+        "empty": "No chat history yet",
+        "emptyHint": "Start a conversation to begin",
+        "today": "Today",
+        "yesterday": "Yesterday",
+        "thisWeek": "This Week",
+        "earlier": "Earlier",
+        "deleteTitle": "Delete this chat?",
+        "deleteDescription": "This will permanently delete this chat session and its diagram. This action cannot be undone.",
+        "recentChats": "Recent Chats",
+        "justNow": "Just now",
+        "searchPlaceholder": "Search chats...",
+        "noResults": "No chats found"
+    },
     "modelConfig": {
         "title": "AI Model Configuration",
         "description": "Configure multiple AI providers and models",