Web app: public frontend + local agent (browser-based casting)

[gluckdev]

Summary

A web version of CastToTV — open a page in any browser (incl. phone) and cast.

Honest browser constraints (why a pure public site can't do it all)

A web page cannot open UDP/SSDP (DLNA discovery), use raw sockets, scan the /24, run ffmpeg/yt-dlp, or SOAP arbitrary IPs (CORS). It can cast a media URL to Chromecast (Google Cast SDK, Chrome/Edge) and to AirPlay (Safari <video>). So the casting/transcoding/discovery has to live in a backend.

Two tiers

Tier 1 — pure public browser (no install):

• Chromecast via the Cast SDK + Safari AirPlay; "paste a URL → cast" only.
• No DLNA, no discovery, no transcode, no yt-dlp, no multi-room.

Tier 2 — public frontend + local agent (full features, recommended):

• The headless CastingController (#) runs as a small local daemon (FastAPI + WebSocket) on the user's PC / Raspberry Pi / home server.
• The hosted single-page app pairs with that agent over the LAN (or a relay) and drives discovery + DLNA/Chromecast/AirPlay + multi-room + radio/cover-art through it.
• Reuses the existing media servers as-is (HTTPServerThread :8766, the streamer servers :8768).
• This is the Jellyfin / Home Assistant model wearing a public URL: the site is public, the casting happens on the user's network via the agent.

Scope

• FastAPI + WebSocket wrapper around CastingController (depends on #).
• SPA frontend: device list (protocol-tagged), source input (file upload / URL / radio), cast / multi-room / pause / seek / stop, live now-playing.
• Pairing flow for the local agent; graceful "Tier 1 only" mode when no agent is present.
• Honest UI messaging about what each tier supports.

Acceptance

• With the local agent running, the web UI discovers and casts to DLNA/Chromecast/AirPlay incl. multi-room and radio.
• Without an agent, the browser can still cast a URL to Chromecast/AirPlay.

Depends on the headless CastingController foundation (#).