Add Vitest test suite

[its-clawdia]

Adds a test suite using Vitest with 17 tests covering:

• Constants — CMD_DISPLAY_ON / CMD_DISPLAY_OFF byte values
• Constructor — throws when Web Bluetooth is unavailable
• connect() — correct BT device filter, GATT connection, lastWeightGrams initialization
• disconnect() — calls gatt.disconnect
• sendCommand() — writes value to the characteristic
• subscribe() — starts notifications, parses weight (Int16/10), computes g/s across events, filters unknown events, handles missing handler gracefully
• unsubscribe() — stops notifications

All Web Bluetooth APIs are mocked; no hardware required.

Run tests with:

npm test