fix: a failed subtitle no longer destroys the download

Two faults behind the 429 in the report.

The subtitle languages were a wildcard: 'en.*' also matches every
machine-translated variant YouTube offers — en-en-US and dozens more —
so each download fired a burst of subtitle requests and earned an HTTP
429. Languages are now named exactly, as '<lang>,<lang>-orig'. Turning
subtitles off now really does request none; it previously still asked
for English.

Worse, yt-dlp exits non-zero if anything at all failed, and a caption it
could not fetch was enough to mark a fully downloaded video as failed
and leave the file orphaned. yt-dlp now runs with --ignore-errors, and
success is judged by whether the media file actually landed rather than
by the exit code.

429 also gets its own message pointing at the sign-in setting, which
raises the limit.

src-tauri/src/downloader.rs

@@ -79,13 +79,15 @@ pub fn parse_progress_line(line: &str) -> Option<Progress> {
 
 /// Arguments for downloading one video. Kept separate from process spawning so
 /// the argument construction is assertable in tests.
+/// `sub_langs` is empty when subtitles are switched off, in which case none
+/// are requested at all.
 pub fn build_args(
     video_id: &str,
     out_template: &str,
     quality: &str,
     sub_langs: &str,
 ) -> Vec<String> {
-    vec![
+    let mut args = vec![
         "-f".into(),
         format_selector(quality),
         "--merge-output-format".into(),
@@ -99,24 +101,50 @@ pub fn build_args(
         "180".into(),
         "--progress-template".into(),
         PROGRESS_TEMPLATE.into(),
-        // Subtitles come along for offline use, including YouTube's
-        // auto-generated ones. They are written beside the video as WebVTT
-        // rather than muxed in: WebKit reads a <track> reliably, whereas
-        // subtitle streams inside an MP4 it largely ignores.
-        "--write-subs".into(),
-        "--write-auto-subs".into(),
-        "--sub-format".into(),
-        "vtt".into(),
-        "--convert-subs".into(),
-        "vtt".into(),
-        "--sub-langs".into(),
-        sub_langs.into(),
+        // A failing subtitle must never take the video with it. Subtitles are
+        // a bonus; the file is the point.
+        "--ignore-errors".into(),
         "--print".into(),
         "after_move:FTPATH %(filepath)s".into(),
         "-o".into(),
         out_template.into(),
-        format!("https://www.youtube.com/watch?v={video_id}"),
-    ]
+    ];
+
+    if !sub_langs.is_empty() {
+        // Subtitles come along for offline use, including YouTube's
+        // auto-generated ones. WebVTT beside the video rather than muxed in:
+        // WebKit reads a <track> reliably and largely ignores subtitle streams
+        // inside an MP4.
+        //
+        // The languages are named exactly. A wildcard like "en.*" also matches
+        // every machine-translated variant YouTube offers — en-en-US, en-de and
+        // dozens more — and asking for all of them earns an HTTP 429.
+        args.extend([
+            "--write-subs".into(),
+            "--write-auto-subs".into(),
+            "--sub-format".into(),
+            "vtt".into(),
+            "--convert-subs".into(),
+            "vtt".into(),
+            "--sub-langs".into(),
+            sub_langs.to_string(),
+        ]);
+    }
+
+    args.push(format!("https://www.youtube.com/watch?v={video_id}"));
+    args
+}
+
+/// The subtitle languages to request for a preference, or empty for none.
+///
+/// Only the language itself and YouTube's "-orig" variant; anything broader
+/// pulls in machine translations by the dozen.
+pub fn sub_langs_for(pref: &str) -> String {
+    if pref.is_empty() || pref == "off" {
+        String::new()
+    } else {
+        format!("{pref},{pref}-orig")
+    }
 }
 
 /// yt-dlp reports the final path via `--print after_move:`, which is more
@@ -194,15 +222,41 @@ mod tests {
 
     #[test]
     fn subtitles_are_requested_including_auto_generated() {
-        let args = build_args("abc", "/tmp/o.%(ext)s", "best", "en.*,nl.*");
+        let args = build_args("abc", "/tmp/o.%(ext)s", "best", "en,en-orig");
         assert!(args.contains(&"--write-subs".to_string()));
         // The auto-generated track is the only one many videos have.
         assert!(args.contains(&"--write-auto-subs".to_string()));
-        assert!(args.contains(&"en.*,nl.*".to_string()));
+        assert!(args.contains(&"en,en-orig".to_string()));
         // WebVTT, because that is what a <track> element can load.
         assert!(args.contains(&"vtt".to_string()));
     }
 
+    #[test]
+    fn no_preference_means_no_subtitle_requests_at_all() {
+        let args = build_args("abc", "/tmp/o.%(ext)s", "best", "");
+        assert!(!args.contains(&"--write-subs".to_string()));
+        assert!(!args.contains(&"--write-auto-subs".to_string()));
+        assert!(!args.contains(&"--sub-langs".to_string()));
+    }
+
+    #[test]
+    fn a_subtitle_failure_must_not_abort_the_video() {
+        // yt-dlp aborts the whole job on the first error without this.
+        assert!(build_args("abc", "/tmp/o.%(ext)s", "best", "en,en-orig")
+            .contains(&"--ignore-errors".to_string()));
+    }
+
+    #[test]
+    fn languages_are_named_exactly_never_as_a_wildcard() {
+        // "en.*" also matches en-en-US and dozens of machine translations,
+        // and requesting them all earns an HTTP 429.
+        let langs = sub_langs_for("nl");
+        assert_eq!(langs, "nl,nl-orig");
+        assert!(!langs.contains('*'));
+        assert_eq!(sub_langs_for("off"), "");
+        assert_eq!(sub_langs_for(""), "");
+    }
+
     #[test]
     fn best_quality_takes_the_highest_available() {
         let args = build_args("abc123", "/tmp/out.%(ext)s", "best", "en.*");