[codex] Add main subtitle list multi-select

[laubonghaudoi]

Summary

Closes #39.

• Adds active-track multi-select state for the main subtitle list, keyed by subtitle UUID.
• Supports plain click, Cmd/Ctrl-click, Shift-click, Escape clear, and Delete selected cues.
• Adds undo-backed selected-cue deletion in one history step.
• Seeds Bulk Offset from the main-list selection when present, while keeping the drawer's own selection after it opens.

Validation

• npm run lint ✅
• npm test ✅ (56 tests)
• npm run build ⚠️ blocked locally because this environment times out fetching Geist and Geist Mono from https://fonts.googleapis.com; curl to the same Google Fonts URL also timed out.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	subtitle-editor	c6d4a42	Commit Preview URL Branch Preview URL	May 24 2026, 10:15 PM