Add waveform cue creation and playhead timing commands

[laubonghaudoi]

Summary

Make the waveform more like a fast captioning workspace by adding cue creation and timing commands around the current playhead. This focuses on high-impact interactions, not a large desktop-style toolbar.

Why this is in scope

The app already uses wavesurfer regions for cue display and drag/resize editing. This issue adds the missing browser-friendly editing operations users expect from a subtitle editor: create a cue from the waveform, set start/end from playback, and move quickly to the next cue.

Initial scope

• Drag empty waveform space to create a new subtitle cue.
• Insert a subtitle at the current playback position.
• Set selected cue start to current playback time.
• Set selected cue end to current playback time.
• Set selected cue end and advance to the next cue.
• Add minimal keyboard shortcuts for the above.

Non-goals

• Shot-change detection.
• Spectrogram mode.
• Silence detection.
• Full user-configurable shortcuts.

Implementation plan

1. Extend subtitle actions.

   • Add action helpers in hooks/use-subtitle-actions.ts for inserting a subtitle at a time range and updating the active cue boundary from seconds.
   • Keep transformations in lib/subtitle-operations.ts where possible.
   • Ensure new cues are inserted chronologically and IDs remain consistent.

2. Track the active/selected cue.

   • Reuse editingSubtitleUuid and current playback matching where possible.
   • If needed, add a small active cue state in app/[locale]/page.tsx or context.
   • Avoid a large selection model in this issue unless #new multi-select work lands first.

3. Waveform interactions.

   • In components/waveform-visualizer/use-waveform-regions.ts, handle empty-space drag to create a region.
   • On create, call a parent callback to create the subtitle in React state, then let region regeneration own the final region.
   • Keep minimum duration at the existing region minimum or a simple default like 1.0s.

4. Commands and shortcuts.

   • Add a compact command group, probably near the waveform or transport controls.
   • Add shortcuts only where they do not conflict with existing editing behavior.
   • Document shortcuts in the existing instruction/shortcut text.
   • Consider A couple of hotkey ideas #22 when choosing keybindings.

5. Validation.

   • Prevent creating cues that cross neighboring cues when overlap clamping is enabled.
   • Clamp start/end at 0 and media duration when known.
   • Preserve undo/redo.

6. Tests.

   • Add pure operation tests for insert/set-boundary behavior.
   • Add component-level tests only if the current test setup can cover the interaction without brittle wavesurfer mocks.

Acceptance criteria

• A user can create a cue by dragging empty waveform space.
• A user can set start/end of the active cue from the current playback time.
• New cues keep chronological ordering and work with undo/redo.
• The feature remains compact and does not turn the app into a complex desktop clone.

Related

• A couple of hotkey ideas #22