fix: 3 review issues from the keyboard preview / onboarding flow

1) Preview chips weren't actually buttons.
   `modeChip` and `localeChip` in `KeyboardPreviewStub` were
   decorative HStacks — no `Button`, no action, no callback. The
   chevron-down glyph made them *look* like pickers, so a user
   tapping them got nothing. The screenshot the user shared
   ("润色 ▾" / "中文(简体) ▾") shows exactly that surface.

   Fix: wrap each chip in a `Button(action: ...)` with
   `.buttonStyle(.plain)`. The stub now takes `modeId`, `localeId`,
   `onModeCycle`, `onLocaleCycle` and the sheet's `cycleMode` /
   `cycleLocale` advance the config:
     - mode cycles [off → transcribe → polish] (mirrors Settings)
     - locale cycles [auto → zh-Hans → zh-Hant → en-US → ja-JP → ko-KR]
   Mid-recording locale switches call `asr.stop()` because ASR
   sessions are bound to the locale they were started with.
   `modeId == "off"` also stops any in-flight recording so the
   disc isn't recording into a mode that won't insert.

   The mode chip's icon also follows the mode (mic.slash /
   mic / wand) as a redundant visual cue, and both chips get
   accessibility labels (preview.modeChip.cycle /
   preview.localeChip.cycle) so VoiceOver users can use them.

2) Onboarding's "Next" stays enabled when local engine is picked
   but no API key is filled in. Root cause: `ProviderConfig.isConfigured`
   checks `!apiKey.isEmpty && !baseURL.isEmpty && !model.isEmpty` —
   it never asks whether the user *needs* a key. The local engine
   (on-device ASR) doesn't round-trip through the LLM, so an
   empty key on the local path is correct, not a configuration gap.

   Fix: short-circuit `isConfigured` to `true` when
   `engineMode == "local"`. The onboarding "Next" button is
   already disabled on the API page when `!isConfigured`; this
   just makes the gate respect the engine choice. New test
   `testIsConfiguredTrueForLocalEngineWithoutAPIKey` locks the
   behaviour in (local → true, cloud → false, flip back).

3) Add the two new accessibility keys to all four
   `Localizable.strings` files (en + zh-Hans, main app + ext)
   so VoiceOver and the cycle button labels resolve in both
   languages.

Build: BUILD SUCCEEDED.
Tests: 22/22 pass (1 new).

🤖 Generated with Claude Code

OSGKeyboardExt/zh-Hans.lproj/Localizable.strings

@@ -109,6 +109,8 @@
 "preview.placeholder" = "试着输入或按 disc 录音";
 "preview.clear" = "清空";
 "preview.openSettingsA11y" = "打开 OSGKeyboard 设置";
+"preview.modeChip.cycle" = "切换输入模式";
+"preview.localeChip.cycle" = "切换识别语言";
 
 /* Keyboard (ext) */
 "keyboard.placeholder.idle" = "按住说话";