feat: Implement server-side read status for cross-device synchronization

Replace localStorage-based message read tracking with server-side storage
to enable unread badge synchronization across all devices and browsers.

Changes:
- Add read_status.py module for server-side read status management
- Add GET /api/read_status endpoint to fetch read status
- Add POST /api/read_status/mark_read endpoint to update read status
- Update app.js to load/save read status from server instead of localStorage
- Update dm.js to load/save DM read status from server instead of localStorage
- Read status stored in MC_CONFIG_DIR/.read_status.json for persistence

Benefits:
- Unread badges sync across all devices (phone, computer, tablet)
- Read status persists across browser sessions
- No more duplicate unread notifications when switching devices

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

README.md

@@ -13,6 +13,7 @@ A lightweight web interface for meshcore-cli, providing browser-based access to
 - 💬 **View messages** - Display chat history with intelligent auto-refresh
 - 🔔 **Smart notifications** - Bell icon with unread message counter across all channels
 - 📊 **Per-channel badges** - Unread count displayed on each channel in selector
+- 🔄 **Cross-device sync** - Unread message status syncs across all devices (server-side storage)
 - ✉️ **Send messages** - Publish to any channel (140 byte limit for LoRa)
 - 💌 **Direct messages (DM)** - Send and receive private messages with delivery status tracking
 - 📡 **Channel management** - Create, join, and switch between encrypted channels