feat(overlay): transcription failure recovery with one-tap retry

[leo-fengchao]

💡 Overview

Gives transcription failures a full recovery path so a single network blip / ASR timeout no longer loses an entire utterance. The audio is retained, the failure is surfaced on the overlay with a retry affordance, and retry replays the saved audio and streams the result back live.

⚠️ Built on top of #20 (native cpal capture). Until #20 merges, this PR's diff also shows #20's commits. Please review/merge #20 first.

🛠️ Key Changes

• Recording retention: the same 16 kHz mono PCM sent to ASR is buffered and written to a WAV on stop. A new keep_recordings setting keeps successful recordings ~31 days; failed ones are kept for retry and reclaimed by the same 31-day sweep, or deleted once a retry succeeds. History gains play / retry entries.
• One-tap retry: failures show a retryable hint + button on the overlay; retry replays the WAV, streams the transcript live, and restores focus to the user's previous app before pasting (clicking the button activates the overlay, so the foreground app is captured and reactivated).
• Keyboard & ESC: while a retryable failure is shown, pressing the main hotkey triggers the retry; ESC terminates both the error and the retrying state. The retry button shows the triggering hotkey, e.g. 重试 (R ⌥), matching the settings-page symbols.
• No-speech quick stop: stopping without speaking ends immediately (ESC-like) instead of waiting on a commit timeout, skipping the leading start-cue window when judging silence.
• ASR robustness: commit_and_await_final returns an error on timeout instead of silently falling back to partial text, so the failure/retry path can engage.

🧪 Testing

• cargo test (246 unit tests) and npx vitest run (123 tests) pass; cargo clippy clean; biome clean.
• New unit tests cover hotkey-label formatting, no-speech audio-signal detection, and history failure/replace records.
• Manually verified on macOS: WAV save/retain/cleanup, overlay/keyboard/history retry, ESC termination, focus restore, and the retry button label.

📸 Screenshots

UI changes (overlay retry button + label, settings "保留录音" toggle, history play/retry entries) — screenshots to be added.

🔗 Related

Depends on #20.