[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).

OSGKeyboardExt/Views/KeyboardRootView.swift

@@ -154,16 +154,19 @@ extension KeyboardRootView {
 #Preview("Keyboard · Idle") {
     KeyboardRootView(state: KeyboardViewController.State.previewIdle)
         .frame(width: 390, height: 280)
+        .preferredColorScheme(.dark)
 }
 
 #Preview("Keyboard · Recording") {
     KeyboardRootView(state: KeyboardViewController.State.previewRecording)
         .frame(width: 390, height: 280)
+        .preferredColorScheme(.dark)
 }
 
 #Preview("Keyboard · Processing") {
     KeyboardRootView(state: KeyboardViewController.State.previewProcessing)
         .frame(width: 390, height: 280)
+        .preferredColorScheme(.dark)
 }
 #endif