Add subtitle statistics and reading-speed warnings

[laubonghaudoi]

Summary

Add local statistics and reading-speed warnings so users can quickly see whether subtitles are readable. This closes a practical gap with desktop tools while staying lightweight and browser-only.

Why this is in scope

The app already computes duration, word count, and character count in the subtitle list. This issue turns that into useful subtitling feedback: CPS, WPM, line length, repeated lines, and track-level summary stats.

Implementation plan

1. Add a pure metrics module.

   • Create lib/subtitle-metrics.ts.
   • Compute per-cue metrics: duration, word count, character count, characters per second, words per minute, line count, max line length, gap before, gap after.
   • Compute track-level metrics: total cues, total words, total characters, total subtitle display time, average duration, max CPS, max WPM, most repeated lines.

2. Define warning thresholds.

   • Use conservative defaults: max CPS 20, max WPM 180, max line length 42, max lines 2, min duration 1.0s, max duration 7.0s.
   • Keep thresholds in one exported config object so the QA tool can reuse them.
   • Avoid language-specific presets in the first version.

3. Add inline cue warnings.

   • Extend the optional stats column or add compact warning badges on subtitle rows.
   • Keep the default UI quiet: warnings should be visible but not dominate editing.
   • Tooltip or popover should explain the reason, e.g. 24 CPS exceeds 20 CPS.

4. Add a statistics dialog.

   • Add a header action, likely near Settings / Find Replace / Bulk Offset.
   • Show track summary, worst offenders, repeated lines, and most frequent words.
   • Provide a simple Download report action as plain text or CSV if straightforward.

5. Keep it integrated with future QA work.

   • Reuse lib/subtitle-metrics.ts from the QA rules engine.
   • Do not duplicate duration/word/char logic inside React components.

6. Tests.

   • Add tests/subtitle-metrics.test.ts.
   • Cover CPS/WPM math, line splitting, repeated lines, empty text, and zero-duration safety.

Acceptance criteria

• Users can inspect track-level statistics without leaving the editor.
• Cue rows can show reading-speed/line-length warnings when enabled.
• Metrics are computed by pure utility functions with unit tests.
• No backend, account, or media-processing dependency is introduced.