Add high-quality scene replacement and flexible video exports

[nickbeau]

Summary

• add on-device MediaPipe presenter segmentation with still-image and looping-video background replacement
• preserve source frame rate and quality with deterministic WebCodecs rendering, temporally stable masks, and original-audio remuxing
• add Original, YouTube 1080p, and Shorts 1080p formats plus subtitle highlight colour and background/outline styling
• restore speech following and smoke tests on Windows ARM64 through a verified x64 speech sidecar

Validation

• npm audit --audit-level=high — 0 vulnerabilities
• npm run speech:model — model installed and available
• npm test — 68 tests passed
• npm run typecheck — passed
• npm run speech:smoke — passed with a finalized local transcript
• npm run build:desktop — passed

Notes

• MediaPipe WASM and segmentation model assets are bundled for offline use.
• Frame-accurate rotoscoping may render slower than real time so frames are never dropped.
• Manual local verification produced 1920x1080 YouTube and 1080x1920 Shorts MP4 files at 30 fps with audio; custom subtitle colour and outline rendering were also verified.