[JJC-20260618-005-A] Review-driven P0 polish

6 review-driven small fixes to the P0 commit series (e0b92ff / e93bab5 /
2e2d8e3 / 013c498). No behavior changes, no new features, no push yet.

Code:
- RED-1: Remove empty init() from OSGKeyboardApp; @StateObject is initialized
  inline and the only thing init() did was print a DEBUG log.
- RED-4: Re-add .preferredColorScheme(.dark) to the 3 keyboard extension
  #Preview blocks. Custom keyboards always render dark; the preview should
  match.
- RED-6: ThemedRoot #Preview blocks now use the new EnvironmentKey
  (\.themePalette) via a private ThemedPreviewContent, instead of hard-coding
  Palette.dark / Palette.light.
- BUG-5: ThemedRoot body now also fills the background with the active
  palette's background color (repeats the colorScheme ternary — accepted
  because body must be a single expression).

Docs:
- DOC-1: CHANGELOG [Unreleased] top note: this is a v0.1.0 → v0.1.1 polish,
  nothing removed, iOS 26 SpeechAnalyzer still 0.2.0.
- DOC-2: README + README.zh.md Limitations: explicit note for iOS 26+ users
  in v0.1.1.

Verified: xcodebuild Debug-iphonesimulator BUILD SUCCEEDED, 8/8 unit
tests pass on iPhone 17 sim (iOS 26.3.1).

README.zh.md

@@ -132,6 +132,7 @@ LLMProvider(
 - iOS 沙盒：键盘扩展 ~60 MB 内存上限，必须开完全访问
 - 密码框与部分 `WKWebView` 输入框不可用（iOS 限制）
 - iOS 18/19 用 `SFSpeechRecognizer` 做端侧 ASR；iOS 26+ 的 `SpeechAnalyzer` 计划下版接入（更快、支持语种更多）
+- v0.1.1 中 iOS 26+ 用户仍走 iOS 18 `SFSpeechRecognizer` 路径；iOS 26 `SpeechAnalyzer` 计划在 0.2.0 接入
 
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