Add transcript import with auto segmentation

[laubonghaudoi]

Summary

• Adds browser-only transcript import from pasted text or .txt files.
• Splits transcript text into draft subtitle cues by lines, sentences, or max-character chunks.
• Shows a preview table before import and imports into an empty active track or a new track.
• Adds localized strings for English, German, and Cantonese.

Closes #37

Verification

• npm run lint passed.
• npm test passed: 56 tests.
• npx biome ci --linter-enabled=false components/transcript-import-dialog.tsx components/app-header/load-srt.tsx components/subtitle/track-tabs.tsx components/subtitle/subtitle-list-empty.tsx lib/transcript-segmentation.ts tests/transcript-segmentation.test.ts messages/en.json messages/de.json messages/yue.json passed.
• npm run build was attempted, but this local environment cannot reach Google Fonts for next/font (Geist and Geist Mono fetches time out/fail), so the build stops before completing.

Notes

• No ASR, backend call, account flow, cloud upload, or .txt drag-and-drop support is added in this PR.
• The repo has no in-repo GitHub Actions preview configuration; any preview deployment should come from the external GitHub/Cloudflare integration.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	subtitle-editor	a15f98d	Commit Preview URL Branch Preview URL	May 10 2026, 03:50 AM