Anti-bot: configurable YouTube player clients (global + per-channel)

YouTube's bot-detection cracks down on different player clients over
time — the `web` client gets captcha-walled, then later `tv`/`mweb` are
the safe ones, etc. Let the user route around whichever client is
currently being targeted instead of being stuck with yt-dlp's pick.

- New backup.youtube_player_clients config (comma-separated, blank =
  yt-dlp default). Maps to --extractor-args youtube:player_client=…
- Per-channel DownloadOptions.youtube_player_clients override (None =
  defer to global). Resolved in the downloader's apply_player_client,
  which omits the flag entirely when empty so yt-dlp keeps its default
  client set (the recommended baseline).
- Threaded into the live downloader at construction + settings save
  (app + web).

UI: a "YouTube player clients" field in both global Settings (desktop +
web) and the per-channel options dialog, with hints pointing at
"tv,mweb" as the current least-bot-checked combo for captcha-prone
channels.

Verified the settings round-trip (global persists to config + echoes;
per-channel override saves + reads back). 87 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/download_options.rs

@@ -85,6 +85,13 @@ pub struct DownloadOptions {
     #[serde(default)]
     pub subtitle_format: Option<String>,
 
+    /// Per-channel YouTube player-client override (comma-separated, e.g.
+    /// "tv,mweb"). `None` defers to the global
+    /// `backup.youtube_player_clients`. Resolved in the downloader, not
+    /// `apply()`, since the global default lives in config.
+    #[serde(default)]
+    pub youtube_player_clients: Option<String>,
+
     /// Raw passthrough — every entry is appended as a separate argument to
     /// yt-dlp. Lets users access any flag we haven't exposed yet. Equivalent
     /// to Tartube's `extra_cmd_string`.