feat: implement Automated Smart-Trim via FFmpeg silencedetect

[samalbishnupriya06-stack]

Summary

Implements Automated Smart-Trim (Phase 1 + Phase 2 + Phase 3 skeleton) as described in issue #<ISSUE_NUMBER>.

Uses the existing FFmpeg WASM pipeline to detect silent segments client-side — no data ever leaves the browser.

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Changes

New Files

• src/hooks/useSmartTrim.ts — state hook for silence detection, segment toggling, and keep-range computation
• src/components/SmartTrimControl.tsx — UI panel with configurable threshold, segment list, Apply/Reset actions
• src/components/SilenceTimeline.tsx — visual overlay on the video timeline showing silence intervals

Modified Files

• src/lib/types.ts — added SilenceSegment and SmartTrimStatus types
• src/lib/ffmpeg.ts — added detectSilence() using FFmpeg's silencedetect filter
• src/components/VideoEditor.tsx — integrated useSmartTrim hook and new components

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How It Works

1. User clicks Detect Silence in the Smart Trim panel
2. detectSilence() runs ffmpeg -af silencedetect=noise=XdB:duration=Ys -f null - on the uploaded file
3. Stdout/stderr is parsed for silence_start / silence_end timestamps
4. Segments appear as purple highlights on the timeline and a reviewable list in the sidebar
5. User toggles individual segments, then clicks Apply Trim (N) to compute keep ranges

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Testing Done

• Tested with a video that has clear silence gaps
• Tested with a video that has no silence (edge case: empty list shown)
• Confirmed no network requests are made during analysis (DevTools → Network)
• Tested Apply Trim logs correct keep ranges to console
• No TypeScript errors (bun run build passes)
• No ESLint errors

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Notes for Reviewers

• Phase 3 integration (feeding keepRanges into the export pipeline for multi-segment concat) is scaffolded in handleSmartTrimApply but left as a follow-up, as it requires changes to the FFmpeg filtergraph concat logic. See the TODO comment in VideoEditor.tsx.
• The detectSilence function creates its own FFmpeg instance to avoid interfering with the main export pipeline. If the project later moves to a shared instance, this can be refactored.

Closes #1146

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