fix: resolve comprehensive review findings (security, concurrency, perf, Android, UI)

Multi-dimension review of v0.8.2. Excludes the deliberately deferred
default_config.yaml weak-default-key item (C1).

Backend:
- calibration: create_default_calibration no longer exceeds led_count for small
  odd counts (bounded trim + regression test)
- game-integration: generic webhook now requires auth_token; constant-time
  compare_digest in all adapters; per-IP failed-auth rate limit on the ingest
  route; auth_token encrypted at rest via secret_box (migration-safe)
- playlist engine: serialize _state/_task under the lifecycle lock to close a
  delete-mid-play race (+ concurrency tests)
- main: stop the calibration session on shutdown (restore prior target)
- home_assistant: validate HA host via the LAN classifier on create/update
- perf: drop slow preview-WS clients instead of blocking the send loop; cache
  composite full-strip resize linspaces; effect_stream lava reuses scratch

Frontend:
- setup/auto-calibration wizard: guard _state after awaits (cancel-safe), await
  session teardown before output start, busy-gate skip-calibration, manual
  display input keeps focus, move focus on step change
- calibration: destroy EntitySelect on modal close
- color-strips test: dirty-flag-gated render + cached ctx/ImageData
- a11y/TV: focus-visible for new wizard/auto-cal/corner controls, aria-labels on
  the spatial corner/edge picker; theme-aware syntax tokens; dead/undefined CSS
  tokens removed; .modal-error styled; i18n titles (en/ru/zh)

Android:
- ApiKeyManager: EncryptedSharedPreferences with verified, data-safe legacy
  migration that never rotates an existing key
- CaptureService: validate MediaProjection token before promoting; satisfy the
  startForeground 5s contract on the bail path
- NotificationListener: connection-scoped executor with lazy fallback
- BLE: request BLUETOOTH_SCAN/CONNECT at runtime + guard handler-thread
  SecurityExceptions
- Root: cancellation-aware su grant probe

Adds 14 tests. Gate: ruff + tsc 5.9.3 + esbuild + pytest (2185 passed) +
compileDebugKotlin all green.

android/app/build.gradle.kts

@@ -210,6 +210,10 @@ dependencies {
     implementation("androidx.core:core-splashscreen:1.0.1")
     // QR code generation for displaying server URL on TV
     implementation("com.google.zxing:core:3.5.3")
+    // EncryptedSharedPreferences (Android Keystore-backed) for the per-install
+    // server API key (see ApiKeyManager). Falls back to plain SharedPreferences
+    // when the keystore is unavailable.
+    implementation("androidx.security:security-crypto:1.1.0-alpha06")
     // USB-serial drivers (CH340, CP2102, FTDI, Prolific, CDC-ACM) for
     // driving Adalight/AmbiLED controllers plugged into Android TV boxes.
     implementation("com.github.mik3y:usb-serial-for-android:3.8.1")