README + Cargo description: cover every shipped feature

The README still described yt-offline as a YouTube-only desktop app.
Rewrite it to reflect what actually exists today:

- Multi-platform support (YouTube, TikTok, Twitch, Vimeo, Bandcamp,
  SoundCloud, Odysee, and yt-dlp's full fallback list) with a table
  of what each platform supports.
- Library, playback, downloads, web server, Plex integration, themes,
  and misc sections — each grouping the relevant features so a reader
  can spot what matters to them.
- Bundled-yt-dlp install walkthrough (venv + curl_cffi + deno).
- Updated config.toml example with every field the app reads.
- Library layout diagram showing the new platform sibling folders
  and the `.source-url` sidecar.
- Usage section split between desktop GUI and web UI.
- Troubleshooting now lists the impersonation error path, missing
  python3-venv, the transcoding-seek caveat, and rate-limit recovery.
- Pointer to SECURITY_AUDIT.md.
- AGPL §13 obligations spelled out.

Also expand the Cargo.toml description so `cargo metadata` and crates.io
listings convey the multi-platform / web UI scope at a glance.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -2,7 +2,7 @@
 name = "yt-offline"
 version = "0.1.0"
 edition = "2021"
-description = "A desktop app for archiving YouTube channels with yt-dlp"
+description = "Self-hosted archive for YouTube, TikTok, Twitch, Vimeo, Bandcamp, SoundCloud, Odysee and more. Desktop GUI + web UI, bundled yt-dlp with curl_cffi impersonation, Plex export, SQLite-backed resume tracking. AGPL-3.0."
 
 [dependencies]
 eframe = "0.29"