feat: implement desktop notifications with comprehensive filtering

Implemented Phase 10 (Desktop Notifications) with three stages:
notify-rust integration, smart filtering, and production polish.

Stage 1 - Base Implementation:
- Add NotificationManager module (src/notifications.rs, 350+ lines)
- Integrate notify-rust 4.11 with feature flag "notifications"
- Implement NotificationsConfig in config.toml (enabled, only_mentions, show_preview)
- Add notification_manager field to TdClient
- Create configure_notifications() method for config integration
- Hook into handle_new_message_update() to send notifications
- Send notifications for messages outside current chat
- Format notification body with sender name and message preview

Stage 2 - Smart Filtering:
- Sync muted chats from Telegram (sync_muted_chats method)
- Filter muted chats from notifications automatically
- Add MessageInfo::has_mention() to detect @username mentions
- Implement only_mentions filter (notify only when mentioned)
- Beautify media labels with emojis (📷 📹 🎤 🎨 📎 etc.)
- Support 10+ media types in notification preview

Stage 3 - Production Polish:
- Add graceful error handling (no panics on notification failure)
- Implement comprehensive logging (tracing::debug!/warn!)
- Add timeout_ms configuration (0 = system default)
- Add urgency configuration (low/normal/critical, Linux only)
- Platform-specific #[cfg] for urgency support
- Log all notification skip reasons at debug level

Hotkey Change:
- Move profile view from 'i' to Ctrl+i to avoid conflicts

Technical Details:
- Cross-platform support (macOS, Linux, Windows)
- Feature flag for optional notifications support
- Graceful fallback when notifications unavailable
- LRU-friendly muted chats sync
- Test coverage for all core notification logic
- All 75 tests passing

Files Changed:
- NEW: src/notifications.rs - Complete NotificationManager
- NEW: config.example.toml - Example configuration with notifications
- Modified: Cargo.toml - Add notify-rust 4.11 dependency
- Modified: src/config/mod.rs - Add NotificationsConfig struct
- Modified: src/tdlib/types.rs - Add has_mention() method
- Modified: src/tdlib/client.rs - Add notification integration
- Modified: src/tdlib/update_handlers.rs - Hook notifications
- Modified: src/config/keybindings.rs - Change profile to Ctrl+i
- Modified: tests/* - Add notification config to tests

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

config.example.toml

@@ -0,0 +1,35 @@
+# Telegram TUI Configuration Example
+# Copy this file to ~/.config/tele-tui/config.toml
+
+[general]
+# Timezone offset (e.g., "+03:00", "-05:00")
+timezone = "+03:00"
+
+[colors]
+# Colors: red, green, blue, yellow, cyan, magenta, white, black, gray
+# Also available: lightred, lightgreen, lightblue, lightyellow, lightcyan, lightmagenta
+incoming_message = "white"
+outgoing_message = "green"
+selected_message = "yellow"
+reaction_chosen = "yellow"
+reaction_other = "gray"
+
+[notifications]
+# Enable desktop notifications for new messages
+enabled = true
+
+# Only notify when you are mentioned (@username)
+only_mentions = false
+
+# Show message preview text in notifications
+show_preview = true
+
+# Notification timeout in milliseconds (0 = system default)
+timeout_ms = 5000
+
+# Notification urgency level: "low", "normal", "critical"
+# Note: Only works on Linux (libnotify), ignored on macOS/Windows
+urgency = "normal"
+
+# Note: Notifications respect Telegram's mute settings
+# Muted chats won't trigger notifications