fix: improve BLE reliability across macOS, Windows, and Linux

[SeanathanVT]

Summary

Addresses unreliable Bluetooth Low Energy device discovery and connection stability caused by Bleak's static BleakScanner methods on macOS CoreBluetooth, as well as metrics not updating correctly after pause/resume cycles. Fixes are cross-platform compatible.

Changes

Device Discovery & Connection

• Replaced static BleakScanner methods with async with BleakScanner() context manager — more stable on macOS CoreBluetooth and consistent across all platforms
• Added 3-retry mechanism with exponential backoff (up to 10s wait) for transient BLE connectivity issues; searches by cached MAC address and device name
• Configurable device name via BLE_DEVICE_NAME constant at top of app.py — no scanner logic changes needed

Event Loop & Task Management

• Proper event loop cleanup — cancels all pending tasks before closing the loop, preventing resource leaks and crashes
• Stats monitor lifecycle fix — global _stats_monitor_task tracks the active monitor; old tasks are cancelled before new ones on resume, cleaned up on disconnect. Fixes metrics-not-updating-after-pause/resume bug
• asyncio.wait_for() with 2s timeout on stats polling prevents hanging

Error Handling

• Try-catch on all async sequences (start_belt(), resume_session()) — failures properly update app state and trigger disconnect handling instead of leaving the app in a hung state
• Thread-safe coroutine execution — error handling on run_coroutine_threadsafe() calls ensures proper state management if queueing fails

API Compatibility

• Bleak version fallback — supports both set_disconn_callback() (newer) and set_disconnected_callback() (older), ensuring compatibility across Bleak 0.19+

Compatibility

Platform	Status
macOS 12+	✅
Windows	✅
Linux	✅
Bleak 0.19+	✅
Python 3.8+	✅

Testing Recommendations

1. Test device discovery with the WalkingPad nearby
2. Test reconnection after device goes out of range
3. Test pause/resume cycles — verify metrics continue updating correctly
4. Test rapid start/pause/resume sequences — verify no duplicate stats tasks
5. Test on multiple platforms (macOS, Windows, Linux) if available