[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.md

@@ -133,6 +133,7 @@ That's it. No other code changes required.
 - iOS sandboxes keyboard extensions: ~60 MB memory cap, Full Access required.
 - The keyboard does **not** work in password fields or some `WKWebView` textareas (iOS limitation).
 - iOS 18/19 ships with `SFSpeechRecognizer` for on-device ASR. iOS 26+ `SpeechAnalyzer` is planned for the next release — it is significantly faster and supports more locales.
+- iOS 26+ users in v0.1.1 use the iOS 18 `SFSpeechRecognizer` path; the iOS 26 `SpeechAnalyzer` is planned for 0.2.0.
 
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