Add link preview cards for URLs in messages

LINK_PREVIEW_IMPLEMENTATION.md

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+# Link Preview Implementation Summary
+
+## Overview
+
+This PR implements link preview functionality for the ArcaneChat Android app, allowing users to see preview cards for shared URLs similar to Telegram and other modern messengers.
+
+## Changes Made
+
+### Core Components
+
+1. **LinkPreview.java** - Data model
+   - Stores URL, title, description, imageUrl, and timestamp
+   - Includes `hasContent()` method to check if preview has displayable data
+
+2. **LinkPreviewFetcher.java** - Metadata fetcher
+   - Fetches Open Graph and HTML metadata from URLs
+   - Respects proxy settings from Delta Chat core (SOCKS5)
+   - Handles HTTP/HTTPS connections with proper error handling
+   - Parses og:title, og:description, og:image with HTML fallbacks
+   - Makes relative image URLs absolute
+   - HTML content size limited to 500KB
+
+3. **LinkPreviewUtil.java** - URL extraction
+   - Regex-based URL detection in message text
+   - Extracts first HTTP/HTTPS URL from text
+   - Improved regex to handle trailing punctuation
+
+4. **LinkPreviewView.java** - UI component
+   - Custom LinearLayout-based view
+   - Displays title, description, image, and domain
+   - Uses Glide for image loading
+   - Clickable card opens URL in browser
+   - Proper intent resolution checking
+
+5. **LinkPreviewCache.java** - Caching system
+   - Thread-safe LRU cache (100 entries)
+   - Singleton with volatile instance field
+   - Prevents redundant network requests
+
+6. **LinkPreviewExecutor.java** - Thread management
+   - Fixed thread pool (2 threads) for fetching
+   - Singleton with volatile instance field
+   - Prevents thread exhaustion
+
+### UI Integration
+
+1. **link_preview_view.xml** - Layout
+   - MaterialCardView with proper styling
+   - ImageView for preview image (120dp height)
+   - TextViews for title, description, domain
+   - Uses theme attributes for colors
+
+2. **conversation_item_sent.xml & conversation_item_received.xml**
+   - Added ViewStub for link preview
+   - Proper margins and positioning
+
+3. **ConversationItem.java** - Integration logic
+   - Added `setLinkPreview()` method
+   - Checks preference setting
+   - Only shows for text messages with URLs
+   - Async fetching with thread pool
+   - Cache checking before fetch
+   - UI updates on main thread
+
+### Settings
+
+1. **preferences_privacy.xml**
+   - Added link preview toggle in Privacy section
+   - Default: enabled
+
+2. **Prefs.java**
+   - Added `LINK_PREVIEWS` constant
+   - Added `areLinkPreviewsEnabled()` method
+   - Added `setLinkPreviewsEnabled()` method
+
+3. **strings.xml**
+   - Added "Link Previews" title
+   - Added explanation text mentioning proxy respect
+   - Added "Link preview image" content description
+
+## Privacy & Security Considerations
+
+✅ **User Control**: Can be disabled in Privacy settings
+✅ **Proxy Support**: Respects SOCKS5 proxy configuration
+✅ **No Tracking**: Generic User-Agent, no tracking headers
+✅ **Size Limits**: 500KB HTML limit to prevent abuse
+✅ **Timeout**: 10s connect, 10s read timeouts
+✅ **Caching**: Minimizes network requests
+✅ **Error Handling**: Graceful failures, no crashes
+
+## Code Quality
+
+All code review feedback addressed:
+- ✅ Thread-safe singletons with volatile
+- ✅ Thread pool instead of Thread creation
+- ✅ Proper error handling (NumberFormatException, etc.)
+- ✅ Intent resolution checking
+- ✅ Correct size calculations
+- ✅ Improved regex patterns
+- ✅ Proper null checking
+- ✅ Documentation comments
+
+## Documentation
+
+- `docs/LINK_PREVIEWS.md` - Full feature documentation
+- Inline code comments
+- This implementation summary
+
+## Testing Recommendations
+
+When build environment is available:
+
+1. **Basic Functionality**
+   - Send message with HTTP URL
+   - Send message with HTTPS URL
+   - Verify preview appears after fetching
+   - Tap preview to open URL
+
+2. **Edge Cases**
+   - Message with multiple URLs (should show first)
+   - URL with query parameters
+   - URL with fragments
+   - Non-English URLs
+   - URLs without previews
+
+3. **Settings**
+   - Disable in Privacy settings
+   - Verify no previews shown when disabled
+   - Re-enable and verify they work again
+
+4. **Proxy**
+   - Configure SOCKS5 proxy
+   - Send message with URL
+   - Verify request goes through proxy
+
+5. **Performance**
+   - Send many messages with URLs quickly
+   - Verify thread pool handles load
+   - Check memory usage
+   - Verify UI remains responsive
+
+6. **Error Handling**
+   - Invalid URLs
+   - Timeout URLs
+   - 404 URLs
+   - Non-HTML content
+   - Large HTML pages
+
+## Known Limitations
+
+1. **HTML Parsing**: Uses regex instead of proper HTML parser (Jsoup)
+   - Trade-off: Simpler, no new dependency
+   - May miss some edge cases with complex HTML
+
+2. **Single URL**: Only shows preview for first URL in message
+   - Could be extended to multiple previews in future
+
+3. **No Video/Audio**: Only fetches static metadata
+   - Could be extended to support video/audio previews
+
+4. **No Size Preference**: Always loads previews
+   - Could add WiFi-only option in future
+
+## Migration Notes
+
+No database changes required. Feature is additive only.
+
+## Performance Impact
+
+- Minimal: Async fetching, caching, thread pool
+- Network: Only fetches when URL present and enabled
+- Memory: LRU cache limited to 100 entries
+- UI: No blocking, updates asynchronously
+
+## Compatibility
+
+- Min SDK: 21 (unchanged)
+- Target SDK: 35 (unchanged)
+- No new dependencies added
+- Uses existing Glide for images
+- Uses Material Design components already in app