What changed

The app now picks the right Bluetooth scanning power automatically based on the situation:

• Low Latency when your AirPods are connected — fast updates while you're using them
• Balanced when the app is open and your headphones aren't connected — same as before
• Low Power when the app is in the background and no headphones are connected — saves battery

The "Scanner mode" setting in General Settings is gone — the app decides based on context. The TroubleShooter still bumps to Low Latency while it's actively scanning.

Also fixes a case where scanning could fail to start for users who hadn't granted the Bluetooth Connect permission, or where Android's headset profile took a while to come up.

Refs #41, #124, #137

Technical Context

• Critical regression fix: the new BleScanModeController.scannerMode combine includes bluetoothManager.connectedDevices, which is stateIn(initialValue = null).filterNotNull() — it never emits if the HEADSET profile proxy stalls or BLUETOOTH_CONNECT is missing. The combine then never produced a mode and BlePodMonitor.createBleScanner() blocked forever. Wrapped that source with .onStart { emit(emptyList()) }.catch { emit(emptyList()) } so the controller emits within one tick and upgrades when real data arrives.
• Refcounted override: replaced setTemporaryOverride / clearTemporaryOverride with suspend fun <T> withTemporaryOverride(mode, block): T. Internal state is MutableStateFlow<Map<ScannerMode, Int>>; exposed mode picks the highest-priority active override (LOW_LATENCY > BALANCED > LOW_POWER). Eliminates the race where one caller's finally could clear another caller's override.
• Reactive bonded set: new BluetoothManager2.bondedDeviceAddresses: Flow<Set<BluetoothAddress>> driven by ACTION_BOND_STATE_CHANGED, replacing a one-shot bondedDevices().first() query inside mapLatest. Receiver-path query is wrapped in try/catch so a thrown SecurityException (e.g. permission revoked at runtime) doesn't escape onReceive.
• TroubleShooter uses withTemporaryOverride(LOW_LATENCY) override@{ ... }. return@override replaces the old return@launch to keep early-exit paths legal through the non-inline lambda.
• Cleanup: ScannerMode.{identifier, labelRes} removed (dead after UI deletion). Five settings_scanner_mode_* keys removed across all 76 locale files. The DataStore key core.scanner.mode retains stale values silently — no migration needed.
• Out of scope: TroubleShooter still creates addressless AppleDeviceProfiles, which the new policy can't match for LOW_LATENCY until the user links a paired address. Pre-existing behavior, deferred.

Review checklist

• BLE scanning starts within ~1s of app launch even before BLUETOOTH_CONNECT is granted (regression guard)
• Bluetooth/ScannerMode log shows BALANCED in foreground without connected profile, LOW_POWER in background, LOW_LATENCY once profiled+bonded headphones connect
• TroubleShooter raises override on entry and clears it on completion / cancellation / exception
• No leftover references to R.string.settings_scanner_mode_* or generalSettings.scannerMode