Implement Fault Tolerance: UI polling timeout and backend FFmpeg detection

Implement Transcode-First stitching and Partial Success handling

Fix stitching hangs: add timeout, normalization and soft failure

fix: golden master repair (ui, caching, stitching)

feat(phase4): separate bridge playback and full movie download

feat(phase3): production architecture - smart stitching & job queue

feat: implement audio bridging, cloud persistence, and creative control

feat: phase 1 & 2 - async architecture, refactoring, and status polling