feat(subtitle_editor): add timestamp panel and full-sentence text edi…

[Evelynzzzz]

…ting

• Add a Timestamp panel (timestamp_editor_dialog.dart) to view/adjust subtitle timestamps while editing.
• Add full-sentence subtitle text editing (edit_text_sheet.dart).
• Wire both into subtitle_editor_controller / subtitle_simple_editor_screen and add l10n strings (en/zh) + a controller test.

[kvnloo]

Useful subtitle-editor surface — timestamp sheet + full-sentence text edit with proportional word rebuild is the right split.

This branch is 1 commit ahead and 230 behind main (compare as of 2026-08-31). Today's HEAD 270c268f is FIX: 修复媒体引擎生命周期竞态. playRange in this PR still does _startPlayheadTicker + _audioEngine.playRangeOnce + finally _stopActivePlayback. Please rebase onto current main and wire preview playback through the post-fix session/lifecycle helpers instead of reintroducing the old pattern. AGENTS.md §6.5 already requires session tokens and drop-stale-callback.

Preview vs pending times (before rebase even): _playStart / _playEnd clip using sentence.startTime / sentence.endTime from snapshot, not the in-sheet _startTime / _endTime. After the user nudges a bound, Play still uses the saved sentence, and if the new start is past the saved end, playRange no-ops on start >= end. Preview should use the pending durations (clamped to audio length).

Also: editSentenceText has tests; updateSentenceTimestamps and playRange do not. assets/icon/unlock.svg is CRLF-only — drop it from the PR.

No other review in the thread; happy to re-read after rebase.

[kvnloo]

Useful subtitle-editor surface — timestamp sheet + full-sentence text edit with proportional word rebuild is the right split.

This branch is 1 commit ahead and 230 behind main (compare as of 2026-08-31). Today's HEAD 270c268f is FIX: 修复媒体引擎生命周期竞态. playRange in this PR still does _startPlayheadTicker + _audioEngine.playRangeOnce + finally _stopActivePlayback. Please rebase onto current main and wire preview playback through the post-fix session/lifecycle helpers instead of reintroducing the old pattern. AGENTS.md §6.5 already requires session tokens and drop-stale-callback.

Preview vs pending times (before rebase even): _playStart / _playEnd clip using sentence.startTime / sentence.endTime from snapshot, not the in-sheet _startTime / _endTime. After the user nudges a bound, Play still uses the saved sentence, and if the new start is past the saved end, playRange no-ops on start >= end. Preview should use the pending durations (clamped to audio length).

Also: editSentenceText has tests; updateSentenceTimestamps and playRange do not. assets/icon/unlock.svg is CRLF-only — drop it from the PR.

No other review in the thread; happy to re-read after rebase.