feat: API key in Keychain + actionable permission-denied UX

Security: API key moves from App Group UserDefaults (plaintext on disk)
to the iOS Keychain. The host app writes in Settings; the keyboard
extension reads before each request. Cross-process sharing is via a new
shared keychain-access-group declared in both targets' entitlements.

UX: when the user denies microphone or speech-recognition permission,
the message becomes a tappable row that opens the host app's settings.
Previously the message said "请到「设置」中允许" but the only way to
actually get there was a top-bar ⚙ button that wasn't obviously
related. Auto-clear (2.4s) is now suppressed for .denied so the user
has time to read it. Re-pressing the mic from .denied re-checks
permission so the user can simply press again after granting.

API Keychain migration
----------------------
- New `OSGKeyboardShared/Services/Keychain.swift` — minimal
  `kSecClassGenericPassword` wrapper for one item
  (service "com.osgkeyboard.apikey", account "current"), backed by
  `kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly` (no iCloud sync).
  `setAPIKey("")` deletes the entry rather than storing an empty
  placeholder so "stored but empty" stays distinguishable from
  "not stored" for the noAPIKey error path.
- `ProviderConfig.apiKey` now reads/writes through Keychain instead
  of UserDefaults. `didSet` skips the round-trip when oldValue equals
  apiKey (init reads Keychain, then assigns — without this guard the
  init write would silently re-write the same value).
- One-shot migration: on first `ProviderConfig.init` after upgrade,
  a legacy `config.apiKey` UserDefaults entry is copied to Keychain
  and removed from UserDefaults. The legacy key is renamed in code to
  `apiKeyLegacy` so future reads of `config.apiKey` from UserDefaults
  would be a bug.
- `AppGroupStore.apiKey` reads from Keychain (was UserDefaults).
- Cross-process sharing: both targets' entitlements gain
  `com.apple.security.keychain-access-groups: ["com.osgkeyboard.shared"]`.
  `com.osgkeyboard.shared` is the first entry in both, so it becomes
  each process's default access group — Keychain queries don't need to
  specify `kSecAttrAccessGroup`.

Permission-denied UX
--------------------
- `KeyboardViewController.pressBegan` now accepts `.denied` and
  `.error` as starting states (previously only `.idle`), so pressing
  the mic after returning from Settings re-checks permission
  without waiting for an auto-clear.
- `scheduleAutoClearError` no longer clears `.denied` — only
  transient `.error` is timed. `.denied` is sticky until the user
  takes action (taps the row → settings, or presses mic → re-check).
- `TranscriptLine` `.denied` case now wraps the text in a Button
  that calls `state.openSettings`, with a `chevron.right` to make
  the affordance obvious. The text was shortened to
  "麦克风被拒绝" / "语音识别被拒绝" so the chevron has room and
  the action isn't implied twice (it was previously both in the
  text and via the top-bar ⚙ button).
- VoiceOver hint on the button: "Opens the OSGKeyboard settings
  page where you can grant microphone or speech recognition access."

Tests
-----
- New `OSGKeyboardTests/KeychainTests.swift` — 6 tests covering
  round-trip, empty-string-deletes, idempotent-delete,
  AppGroupStore-reads-from-Keychain, legacy UserDefaults → Keychain
  migration, and "Keychain wins when both are present".
- `LLMClientTests` setUp/tearDown now wipes the Keychain
  (`try? Keychain.deleteAPIKey()`) and clears `StubURLProtocolStorage`
  so tests are independent across runs in the same simulator process.
- All 21 tests pass (6 new + 15 existing).

🤖 Generated with Claude Code

OSGKeyboardExt/KeyboardViewController.swift

@@ -125,7 +125,16 @@ public final class KeyboardViewController: UIInputViewController {
     // MARK: - Press handlers
 
     private func pressBegan() {
-        guard state.phase == .idle else { return }
+        // Allow re-entry from `.denied` and from a finished/cleared
+        // `.error` so the user can simply press the mic again after
+        // returning from Settings with permission granted — they
+        // shouldn't have to wait for an auto-clear timer.
+        switch state.phase {
+        case .idle, .denied, .error:
+            break
+        default:
+            return
+        }
         guard state.mode != .off else { return }
         // Set the intermediate phase SYNCHRONOUSLY so a rapid second
         // press (before the first Task has had a chance to flip phase to
@@ -138,7 +147,6 @@ public final class KeyboardViewController: UIInputViewController {
             let micGranted = await self.permissions.requestMicPermission()
             guard micGranted else {
                 self.state.phase = .denied(.mic)
-                self.scheduleAutoClearError()
                 return
             }
             // iOS 18 SFSpeechRecognizer path: we explicitly ask for Speech
@@ -152,7 +160,6 @@ public final class KeyboardViewController: UIInputViewController {
             let speechGranted = await self.permissions.requestSpeechPermission()
             guard speechGranted else {
                 self.state.phase = .denied(.speech)
-                self.scheduleAutoClearError()
                 return
             }
             self.startPipeline()
@@ -357,11 +364,12 @@ public final class KeyboardViewController: UIInputViewController {
         Task { @MainActor [weak self] in
             try? await Task.sleep(nanoseconds: 2_400_000_000)
             guard let self else { return }
-            // Clear both .error (transient error message) and .denied
-            // (permission was rejected — show the message, then return
-            // to idle so the user can navigate away).
+            // Only transient errors auto-clear. `.denied` is sticky: the
+            // user needs the message long enough to read it AND decide
+            // whether to tap "去设置" or tap the mic to retry. They
+            // dismiss it implicitly by doing either of those things.
             switch self.state.phase {
-            case .error, .denied:
+            case .error:
                 self.state.phase = .idle
             default:
                 break