[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).

OSGKeyboardShared/DesignSystem/ThemedRoot.swift

@@ -21,31 +21,36 @@ public struct ThemedRoot<Content: View>: View {
     public var body: some View {
         content()
             .environment(\.themePalette, colorScheme == .dark ? Palette.dark : Palette.light)
+            .background(colorScheme == .dark ? Palette.dark.background : Palette.light.background)
     }
 }
 
 #if DEBUG
 #Preview("ThemedRoot · Dark") {
     ThemedRoot {
-        ZStack {
-            Palette.dark.background.ignoresSafeArea()
-            Text("Dark")
-                .foregroundStyle(Palette.dark.textPrimary)
-                .font(.title)
-        }
+        ThemedPreviewContent(title: "Dark")
     }
     .preferredColorScheme(.dark)
 }
 
 #Preview("ThemedRoot · Light") {
     ThemedRoot {
-        ZStack {
-            Palette.light.background.ignoresSafeArea()
-            Text("Light")
-                .foregroundStyle(Palette.light.textPrimary)
-                .font(.title)
-        }
+        ThemedPreviewContent(title: "Light")
     }
     .preferredColorScheme(.light)
 }
-#endif
+
+private struct ThemedPreviewContent: View {
+    @Environment(\.themePalette) private var palette
+    let title: String
+
+    var body: some View {
+        ZStack {
+            palette.background.ignoresSafeArea()
+            Text(title)
+                .foregroundStyle(palette.textPrimary)
+                .font(.title)
+        }
+    }
+}
+#endif