feat: iCloud settings sync and cold-start return redesign

- Add iCloud key-value settings sync (engine/language/polish/Flow prefs);
  API keys stay on-device. New "Sync settings via iCloud" toggle.
- Redesign cold-start handoff: bottom-bar left-to-right swipe guidance,
  auto-dismiss on app switch, tap-anywhere to close, retained return link.
- Harden keyboard->app handoff with host-disconnected hint.
- Include prior Unreleased ASR fixes (route-change crash, fallback warning,
  multi-utterance recognition, local ASR diagnostics).

Release 0.5.0 (build 18).

OSGKeyboardExt/zh-Hans.lproj/Keyboard.strings

@@ -139,6 +139,7 @@
 "keyboard.flow.startingSession" = "正在启动语音会话…";
 "keyboard.flow.transcribing" = "识别中…";
 "keyboard.flow.resultTimeout" = "等待识别结果超时，请重试";
+"keyboard.flow.hostDisconnected" = "语音会话已断开，请打开 OSGKeyboard 重新启动";
 "keyboard.flow.manualOpenHost" = "无法自动跳转，请从主屏幕打开 OSGKeyboard 后返回";
 
 /* Keyboard status badges */