Per-channel "skip auth check" toggle for the youtubetab warning

yt-dlp prints a scary-looking ERROR when it can't confirm a channel
tab loaded authenticated:

  ERROR: [youtube:tab] @Channel: Playlists that require authentication
  may not extract correctly without a successful webpage download…
  pass "--extractor-args youtubetab:skip=authcheck" to skip this check

For PUBLIC channels that's just noise — there's no auth-gated content
to miss, and the flag silences it without changing which videos are
found. But for members-only/private channels archived with cookies,
the warning is a real "your cookies may not be working, you might be
getting an incomplete list" signal worth keeping.

So make it a per-channel toggle rather than a blanket setting:

- DownloadOptions gains `skip_auth_check: bool`; apply() emits
  `--extractor-args youtubetab:skip=authcheck` when set. (Its own flag;
  the youtubetab: namespace is distinct from the POT provider's
  youtubepot-bgutilhttp: one, so they coexist.)
- Desktop + web channel-options dialogs gain a checkbox with a tooltip
  spelling out the public-vs-private trade-off.

1 new test; 81 pass.

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

src/app.rs

@@ -219,6 +219,7 @@ struct ChannelOptionsForm {
     quality_idx: usize,           // 0=default, 1=Best, 2=1080p, 3=720p, 4=480p, 5=360p
     audio_only: bool,
     fetch_comments: bool,
+    skip_auth_check: bool,
     limit_rate_kb: String,
     min_filesize_mb: String,
     max_filesize_mb: String,
@@ -440,6 +441,7 @@ impl App {
             quality_idx,
             audio_only: opts.audio_only,
             fetch_comments: opts.fetch_comments,
+            skip_auth_check: opts.skip_auth_check,
             limit_rate_kb: num(opts.limit_rate_kb),
             min_filesize_mb: num(opts.min_filesize_mb),
             max_filesize_mb: num(opts.max_filesize_mb),
@@ -470,6 +472,7 @@ impl App {
             quality,
             audio_only: f.audio_only,
             fetch_comments: f.fetch_comments,
+            skip_auth_check: f.skip_auth_check,
             limit_rate_kb: parse_num(&f.limit_rate_kb),
             min_filesize_mb: parse_num(&f.min_filesize_mb),
             max_filesize_mb: parse_num(&f.max_filesize_mb),
@@ -1904,6 +1907,11 @@ impl App {
                         .on_hover_text("Slow on popular videos. Once captured, comments are browsable from the player modal in the web UI.");
                     ui.end_row();
 
+                    ui.label("Skip auth check");
+                    ui.checkbox(&mut form.skip_auth_check, "Suppress the channel-tab authentication warning")
+                        .on_hover_text("Passes --extractor-args youtubetab:skip=authcheck. Safe for PUBLIC channels — it silences yt-dlp's \"playlists that require authentication may not extract correctly\" warning without changing which videos are found. Leave OFF for members-only/private channels, where that warning signals your cookies may not be working.");
+                    ui.end_row();
+
                     ui.label("Bandwidth cap (KB/s)");
                     ui.add(egui::TextEdit::singleline(&mut form.limit_rate_kb).desired_width(100.0).hint_text("off"));
                     ui.end_row();