Add persistent conversation feature with cross-platform file restoration

[Copilot]

Users must re-import WhatsApp ZIP files on every app launch, losing bookmarks and transcriptions. This PR implements persistent conversation storage that remembers chat sessions, bookmarks, and transcriptions across restarts.

Core Implementation

Persistence Module (src/lib/persistence.svelte.ts)

• IndexedDB storage via idb-keyval for chat metadata (~1MB, never stores ZIP content)
• Hybrid file reference strategy:
  • Electron: Absolute file paths with existence checks
  • Web (Chromium): FileSystemFileHandle stored in IndexedDB
  • Web (Firefox/Safari): Metadata-only with reselect fallback
• Multi-signal validation: message count, timestamps, first message IDs (handles iOS exports without titles)

UI Components

• RestoreSessionModal.svelte: Multi-chat selection with WhatsApp-themed design
• ReselectFileModal.svelte: Drag-drop file picker with validation
• Toast.svelte: User feedback for persistence operations
• Responsive design with mobile-first approach (modal sizing, spacing, touch targets)

Electron Integration

// electron/main.cjs
ipcMain.handle('fs:fileExists', (event, filePath) => fs.existsSync(filePath));
ipcMain.handle('file:readFromPath', async (event, filePath) => {
  const buffer = await fs.promises.readFile(filePath);
  return { buffer, name: path.basename(filePath), success: true };
});

File System Access API

// Request handle with user gesture (toggle "Remember Conversation")
const handle = await promptForFileHandle(file);
await storeFileHandle(handleId, handle);

// Restore with automatic permission check
const permission = await verifyHandlePermission(handle, false, true);
if (permission === 'granted') {
  const file = await handle.getFile();
  // Restore automatically
}

User Flow

First Save:

1. User drops ZIP file → chat loads
2. Toggle "Remember Conversation" → file picker prompts once for persistent access
3. Metadata + file reference stored

Subsequent Opens:

• Electron: Reads from stored file path automatically
• Chrome/Edge: Uses stored FileSystemFileHandle if permission granted
• Firefox/Safari: Shows reselect modal (no File System Access API)

Known Issues

File restoration currently shows reselect modal in all cases. Debug guidance in AI_HANDOFF.md. Root cause likely in file reference capture during initial save or metadata update after handle acquisition.

i18n

Added 11 new strings across 10 languages (de, en, es, fr, it, ja, nl, pt, ru, zh). Non-English translations use English placeholders pending machine translation.

Original prompt

Persistent Conversation Feature

Problem Statement

Resolves #65

Users currently have to re-import their WhatsApp ZIP files every time they open the app. This is inconvenient, especially when users have bookmarks and transcriptions they want to preserve.

Solution Overview

Implement a "Remember Conversation" feature that allows users to persist chat sessions across app restarts without storing large ZIP files in the browser.

Storage Strategy (Hybrid Approach)

Platform	Method	Fallback
Electron	Store absolute file path, read from disk on restore	Prompt to re-select if file missing
Web (Chromium - Chrome/Edge/Opera)	Store FileSystemFileHandle in IndexedDB	Prompt to re-select if permission denied
Web (Firefox/Safari)	Store metadata only	Always prompt to re-select file

What Gets Stored (Always Small ~1MB)

• Chat metadata (title, filename, timestamps, message count)
• Bookmarks array
• Transcriptions map (messageId → transcription text)
• Settings (language, autoLoadMedia, perspective)
• File reference (path/handle/reselect-required marker)

The actual ZIP file content is NEVER stored in the browser.

Technical Implementation

1. New Dependency

Add idb-keyval for simple IndexedDB key-value storage.

2. New Files to Create

src/lib/persistence.svelte.ts

Core persistence module with:

interface PersistedChatMetadata {
  id: string;                      // Unique ID (crypto.randomUUID)
  fileName: string;                // Original filename
  chatTitle: string;               // Parsed chat title (for display)
  messageCount: number;            // For validation
  firstMessageTimestamp: string;   // ISO string - for validation  
  lastMessageTimestamp: string;    // ISO string - for validation
  firstMessageIds: string[];       // First 5 message IDs (backup validation for iOS)
  savedAt: string;                 // ISO timestamp
  updatedAt: string;               // ISO timestamp (for sorting)
  
  // File reference (varies by platform)
  fileReference: 
    | { type: 'electron-path'; filePath: string }
    | { type: 'file-handle'; handleId: string }  // Handle stored separately
    | { type: 'reselect-required' };
  
  // Persisted data (always stored - small)
  bookmarks: Bookmark[];
  transcriptions: Record<string, string>;
  settings: {
    language: string;
    autoLoadMedia: boolean;
    perspective: string | null;
  };
}

Key functions:

• savePersistedChat(chat, file, bookmarks, transcriptions, settings) - Save a chat for persistence
• getPersistedChats() - Get all persisted chats (sorted by updatedAt)
• removePersistedChat(id) - Remove a persisted chat
• restoreChat(persistedChat) - Attempt to restore a chat (handles all platform logic)
• validateRestoredFile(parsed, saved) - Validate a file matches saved metadata
• isFileSystemAccessSupported() - Feature detection for File System Access API
• requestPersistentStorage() - Request persistent storage to prevent eviction

src/lib/components/RestoreSessionModal.svelte

Modal shown on app load when persisted chats exist:

• Lists all persisted chats sorted by updatedAt (most recent first)
• Checkboxes to select which chats to restore
• Shows chat title, last opened date, message count
• "Restore Selected" and "Start Fresh" buttons
• "Don't show this again" checkbox (skips modal but preserves data)

src/lib/components/ReselectFileModal.svelte

Modal prompting user to re-select a file when needed:

• Shows expected filename
• File drop zone / file picker
• Validates selected file against saved metadata
• Shows confirmation if validation confidence is low
• Shows error if validation fails, allows retry
• "Skip" button to skip this chat

3. Files to Modify

src/lib/components/ChatList.svelte

Add "Remember Conversation" toggle to the existing context menu:

┌─────────────────────────────┐
│ ☑ Auto-load Media           │
│ 🌐 Transcription Language → │
│ ─────────────────────────── │
│ 💾 Remember Conversation    │  ← NEW toggle
│ ─────────────────────────── │
│ 🗑️ Remove Chat              │
└─────────────────────────────┘

When toggled ON:

• Save metadata + bookmarks + transcriptions + settings to IndexedDB
• Store file reference based on platform
• Show brief toast: "Conversation will be remembered"

When toggled OFF:

• Remove from IndexedDB
• Show brief toast: "Conversation removed from saved sessions"

src/routes/+page.svelte

• On app load, check for persisted chats
• If found and "don't show again" not set, show RestoreSessionModal
• Handle restoration flow including re-select prompts
• Track which chats are "remembered" to show toggle state in ChatList
• Pass file handle/reference when saving for persistence

src/lib/bookmarks.svelte.ts

Add method to get bookmarks for a specific chat:

getBookmarksForChatAsExport(chatId: string): Bookmark[]

`s...

This pull request was created from Copilot chat.

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