Improve timeline, preview, and mixed caption workflows

[yxmnshmily]

Summary

This Draft PR contains the complete upstream-safe timeline / preview / inspector contribution batch:

• Fit the timeline to clips and captions, show live per-clip duration, visualize same-track overlaps, and confirm destructive track deletion.
• Add drag-safe seeking, read-only hover preview, immediate hover cleanup when playback/gestures start, muted-clip state, and lower idle Player polling.
• Add direct preview-canvas selection, candidate cycling, move/scale/rotate handles, keyboard nudge, shared contain-aware preview geometry, static opacity, and corner radius rendering.
• Connect caption selection to Preview and Inspector, including text/style/transform controls and mixed clip+caption selection/movement/copy-paste.
• Accept Finder video/image/audio/SRT/VTT/TXT drops directly on compatible lanes with placeholder-first placement, background completion, reflow, and failure cleanup.
• Add clip relink, multi-selection “Add to AI”, and timeline/caption drag-to-composer references.
• Add blank-track context workflows: insert, close gaps, select all, clear, visibility/mute/lock/delete, caption style/translation, and audio ducking drill-ins.
• Add selected media to the timeline as one contiguous batch and keep created clips jointly selected.

Temporary dependency on #33

This branch temporarily includes the caption commits from #33 because mixed clip/caption parent wiring must compile against main.

Recommended merge order: merge #33 first, then rebase this branch onto updated main and drop duplicate caption commits before marking this PR ready. The PR base intentionally remains main.

Verification

• Full npm test: PASS.
• npm run build: PASS (existing chunk-size/dynamic-import warnings only).
• Focused timeline-preview suite and new relink/context/drop/preview-transform tests: PASS.
• oxlint: no errors; only existing hook-dependency warnings.
• git diff --check: PASS.
• Delegated Open Code Review completed; no High/Critical findings remain.
• Secret/private scan found no TravelChatCut identity, local paths, map-provider keys, private media, or runtime data.

Intentionally excluded

• TravelChatCut branding, tourism/map workflows, local launcher/runtime integration, credentials, user projects, media, and caches.
• Product-specific forced 9:16/default-track policy and automatic empty-track deletion.
• Private Electron-only behavior and unrelated transcript/product changes.

Visual evidence

The PR stays Draft until a clean upstream-branded Electron instance is used for final visual acceptance. Automated checks support the contribution but do not replace that UI verification.