Add Premiere-style editor: multi-track timeline, client-side preview, MoviePy export

[Ariel-Gal]

Summary

Phase 1 of the PyMontage → web NLE transformation (spec: docs/superpowers/specs/2026-07-05-editor-timeline-design.md, plan: docs/superpowers/plans/2026-07-05-editor-timeline.md).

• /editor — new Premiere-style dark UI: media pool, program monitor, inspector/automation sidebar, multi-track timeline (V2/V1/A1/A2). Vanilla ES modules + Tailwind CDN, no build step. Old slideshow UI at / untouched.
• Project JSON model — browser-owned single source of truth with undo/redo (50 deep) and localStorage autosave.
• Timeline — drag/move with snapping, edge trim, split (S), delete, cross-track moves, Ctrl+wheel zoom, client-side Web Audio waveforms, track hide/mute.
• Monitor — client-side approximate playback (topmost clip wins), J-K-L, frame stepping, timecode.
• Media API — upload with probe + thumbnails, Range-request streaming, per-session storage, offline detection after server restart.
• Export — timeline_renderer.py renders the project JSON via MoviePy (reuses NVENC detection); background job with progress polling, one export at a time.
• Fix — emoji print()s no longer crash on non-UTF-8 Windows consoles.

Automation features (auto-montage, smart trim, auto-slideshow, aspect presets) are stubbed in the sidebar for phase 2.

Test plan

• pytest — 8 tests: renderer (2), media API (3), export API (3)
• Manual browser verification: upload → drag to timeline → trim/split/undo → inspector edits → export downloads correct MP4

🤖 Generated with Claude Code