Release the Local Studio three-step workflow

[judefluen-coder]

What changed

• ships the local-first FastAPI, SQLite, worker, and FFmpeg studio workflow
• turns the editor into a clear vertical three-step flow: select clips, arrange sequence, export video
• adds persistent jobs, subtitle reprocessing, render profiles, delivery checks, and recovery states
• adds a GitHub Pages workflow demo using the same frontend
• unifies yt-dlp detection, YouTube search, and authorized download execution
• simplifies discovery and export UI while preserving advanced controls
• updates README, screenshots, release docs, troubleshooting, and the verification report

Why

The project is moving from a prototype into a practical local tool that collaborators can understand and run. The main path is now explicit, dependencies have actionable diagnostics, and the static demo explains the workflow without requiring media processing.

Validation

• 49 frontend/script tests passed
• 50 backend tests passed
• production and Pages builds passed
• real YouTube search returned video metadata and thumbnail
• real FFmpeg H.264/AAC subtitle-burned export passed
• Playwright desktop and 390px mobile flows passed with zero console errors
• GitHub Pages deployment passed