[P0-③] API Key data flow fix

- AppGroup.defaults: in DEBUG, missing App Group is a hard fatalError
  with a precise remediation message (was a soft print + .standard
  fallback, which desynced the keyboard extension from the main App).
  Release keeps the fallback + NSLog so end-users still get a usable app.
- KeyboardViewController.loadPersistedLocale now prints a masked DEBUG
  view of the live App Group config (provider, baseURL, masked key,
  model, mode, locale) so the extension's view is visible in the
  device console.
- KeyboardViewController.handleFinalTranscript now routes by typed error:
    noAPIKey  → red error '未配置 API Key · 请在主 App 设置中填写'
    http 401  → red error 'API Key 无效 (401) · 请检查主 App 设置'
    http 429  → red error 'API 限流 (429) · 请稍后再试'
    other     → insert raw transcript + generic error badge
- APISettingsCard gains a 'Test connection' button that runs a single
  client.polish('ping') round-trip and surfaces the typed result inline.
- PolishingService.timeout raised 12s → 15s to match LLMClient.request
  timeout (was racing and discarding successful responses in 12–15s).
- Tests: 4 new cases (HTTP 429, transport timeout, App Group cross-process,
  AppGroupStore→LLMClient noAPIKey). All 8 tests pass on iPhone 16e sim.

xcodebuild iOS Simulator: SUCCEEDED
xcodebuild test: 8/8 passed

OSGKeyboardExt/Services/PolishingService.swift

@@ -18,7 +18,7 @@ public actor PolishingService {
     private let store: AppGroupStore
     private let timeout: TimeInterval
 
-    public init(store: AppGroupStore = AppGroupStore(), timeout: TimeInterval = 12) {
+    public init(store: AppGroupStore = AppGroupStore(), timeout: TimeInterval = 15) {
         self.store = store
         self.timeout = timeout
     }