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/commands.rs

@@ -121,6 +121,11 @@ fn explain_yt_dlp_error(stderr: &str, signed_in: bool) -> String {
                 .into()
         };
     }
+    if stderr.contains("429") || stderr.contains("Too Many Requests") {
+        return "YouTube is rate-limiting this machine. Wait a few minutes, or sign in \
+                under Settings → Sign in to YouTube, which raises the limit."
+            .into();
+    }
     if stderr.contains("Operation not permitted") && stderr.contains("Safari") {
         return "macOS blocked access to Safari's cookies. Give FlightTube Full Disk                 Access in System Settings → Privacy & Security, or pick a different                 browser."
             .into();
@@ -1038,15 +1043,15 @@ pub async fn download_video(
     let stderr_lines = stderr_task.await.unwrap_or_default();
     drop(permit);
 
-    if status.success() {
-        // yt-dlp normally reports the path via `--print after_move:`; if that
-        // line went missing, find the file it wrote by its embedded video id.
-        let path = match final_path {
-            Some(p) => p,
-            None => find_by_video_id(&library, &video_id)
-                .await
-                .ok_or("Download finished but the file could not be located.")?,
-        };
+    // yt-dlp exits non-zero if *anything* failed, including a subtitle it could
+    // not fetch. If the video itself landed, the download succeeded — throwing
+    // away a finished file over a missing caption would be absurd.
+    let landed = match final_path {
+        Some(p) if tokio::fs::metadata(&p).await.is_ok() => Some(p),
+        _ => find_by_video_id(&library, &video_id).await,
+    };
+
+    if let Some(path) = landed {
         // YouTube's captions arrive pinned to the left edge and full of
         // karaoke timing tags; clean them before they reach the player.
         tidy_subtitles(&library, &video_id).await;