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).

OSGKeyboardShared/zh-Hans.lproj/Shared.strings

@@ -7,6 +7,7 @@
 /* v0.2.0: flow-level warnings surfaced alongside the final transcript. */
 "flow.warning.cloudPolishMissingKey" = "云端润色需要先在设置中填写 API Key，本次已插入原始识别结果。";
 "flow.warning.localPolishUnavailable" = "内置润色暂不可用，本次已插入原始识别结果。";
+"flow.warning.polishDegraded" = "弱网识别，本次未润色，已插入原始识别结果。";
 
 /* LLM providers */
 "provider.openai" = "OpenAI";