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/OSGKeyboardExt.entitlements

@@ -6,5 +6,9 @@
 	<array>
 		<string>group.com.osgkeyboard.shared</string>
 	</array>
+	<key>com.apple.security.keychain-access-groups</key>
+	<array>
+		<string>com.osgkeyboard.shared</string>
+	</array>
 </dict>
 </plist>