Add main subtitle list multi-select and selected-cue actions

[laubonghaudoi]

Summary

Add first-class multi-select to the main subtitle list so users can apply operations to selected cues instead of only the whole active track or dialog-local selections.

Why this is in scope

Many high-value subtitle tools need a selection model: QA fixes, text cleanup, retiming, deleting ranges, and selected-cue export. The app already has checkbox/range selection patterns inside Bulk Offset and Find / Replace; this issue brings that capability into the main editor.

Initial scope

• Click selects the active cue as today.
• Cmd/Ctrl-click toggles cues in the selection.
• Shift-click selects a range.
• Escape clears selection.
• Delete removes selected cues after confirmation or with undo-friendly behavior.
• Tools can receive selected UUIDs when present.

Non-goals

• Collaborative selections.
• Persistent named selections.
• Spreadsheet-style cell editing.

Implementation plan

1. Add selection state.

   • Add active-track selection state in context/subtitle-context.tsx or a focused hook if keeping context smaller is preferable.
   • Store selected subtitle UUIDs, not numeric IDs.
   • Clear or prune selection when track changes, subtitles are deleted, or a track is reloaded.

2. Update subtitle list interactions.

   • Extend components/subtitle/subtitle-list.tsx and subtitle-item.tsx.
   • Handle Cmd/Ctrl-click and Shift-click without breaking current click-to-scroll behavior.
   • Add selected-row styling that remains distinct from the currently playing cue.

3. Add selected-cue actions.

   • Delete selected cues.
   • Apply bulk offset to selected cues by default when opening Bulk Offset.
   • Let future QA/text tools target selected cues by default.

4. Keyboard behavior.

   • Escape clears selection.
   • Delete removes selected cues when not editing text.
   • Arrow navigation should keep working without accidentally expanding selection in the first pass.

5. Tests.

   • Unit test selection reducer/helper logic if extracted.
   • Add React tests for range/toggle behavior if feasible with the current setup.
   • Test that deleting selected cues preserves ordering and undo history.

Acceptance criteria

• Users can select multiple cues directly in the main subtitle list.
• Selected cues can be deleted or passed into existing bulk tools.
• Selection is UUID-based and robust across reorder/delete operations.
• Existing single-click editing/playback behavior remains intact.