Add multi-platform support: TikTok, Twitch, Vimeo, Bandcamp, SoundCloud, Odysee

yt-dlp already speaks ~1,800 sites; the changes here teach the rest of
the app to route them through platform-specific output folders and
remember where each creator was downloaded from.

New `platform` module
- `Platform` enum: YouTube, TikTok, Twitch, Vimeo, Bandcamp, SoundCloud,
  Odysee, Other.
- `classify_url` returns both the source platform and a `UrlKind`
  (Channel / Playlist / Video / Unknown) — per-platform path parsing
  for each supported host.
- `platform_root(channels_root, platform)` returns the on-disk folder
  per platform. YouTube keeps the legacy `channels/` for backward
  compat; the others live as siblings (`tiktok/`, `twitch/`, …).
- `write_source_url` / `read_source_url` persist the originating URL
  in a `.source-url` sidecar at each creator folder so re-checks no
  longer rely on the YouTube-only "UC + 22 chars" heuristic.

Downloader
- `Downloader::start` now takes `&UrlInfo` instead of `&UrlKind` and
  routes the yt-dlp `-o` template into the right platform folder.
- For channel downloads it also writes `.source-url` so future
  re-checks recover the exact URL.
- Output template uses `%(uploader,channel,creator|Unknown)s` so
  non-YouTube sites that don't expose `channel` still get a sensible
  creator folder.
- Per-platform `archive.txt` keeps cross-platform IDs (TikTok numeric
  vs YouTube base64) from colliding.
- New `recheck_url(&Channel)` prefers `source_url` and falls back to
  the legacy YouTube heuristic for libraries created before this
  change.

Library scanner
- `scan_channels` now walks every platform's folder and tags each
  `Channel` with its `platform` + `source_url`. Sort key puts the
  platform first, then name, so the sidebar groups cleanly.
- `Channel` gains `platform: Platform` and `source_url: Option<String>`
  fields, populated from the on-disk sidecar.

Web UI
- `WebState` gains `library_root` (= parent of channels_root). The
  `/files/` ServeDir is mounted at library_root so non-YouTube videos
  resolve at `/files/<platform>/<creator>/<file>`. Existing YouTube
  URLs still work — they're now `/files/channels/...` instead of
  `/files/...` and the server-rendered JSON updates accordingly.
- Maintenance scan + remove use `library_root` so non-YouTube content
  is included.
- `ChannelInfo` JSON exposes `platform`, `platform_label`,
  `platform_icon`, and `source_url`.
- Sidebar groups channels by platform with each platform's icon.
- "Check for new videos" now uses the stored `source_url` (with the
  YouTube heuristic as a last-resort fallback).
- Download dialog preview shows the detected source + destination
  folder.

Desktop UI
- App gains a `library_root` field mirroring the web side; maintenance
  ops scan/remove against it.
- Sidebar channel labels are prefixed with the platform icon for
  non-YouTube channels; tooltip carries the platform name.
- Download URL field surfaces both source platform and folder path
  preview as you type.

Plex
- Non-YouTube creators get their show folder prefixed with the
  platform name (e.g., `TikTok - cooluser`) so a YouTube channel and
  a TikTok account with the same name don't collide.

Backward compatibility
- Existing YouTube libraries (`channels/<handle>/...`) are untouched.
  New platform folders are created on first run as siblings.
- Channels without a `.source-url` (i.e., everything that pre-dates
  this commit) fall through to the YouTube heuristic, preserving the
  existing re-check behavior.

Tests
- 14 new `platform` tests cover URL classification per platform,
  channel handle extraction, and `platform_root` layout. The old
  `detect_url_kind` / `extract_after` tests move into platform.rs
  since the canonical implementation lives there now.
- 41 unit tests pass.

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

src/main.rs

@@ -19,6 +19,7 @@ mod database;
 mod downloader;
 mod library;
 mod maintenance;
+mod platform;
 mod plex;
 mod stats;
 mod theme;