fix: point audio download links at the directory the server used (closes #533)

The server picks the download directory on download_type alone
(ytdl.py:1530: AUDIO_DOWNLOAD_DIR if download_type == 'audio'). The UI
picked the URL base on download_type *or* a .mp3 extension, so the two
disagreed for any mp3 produced under a video-type download -- a
postprocessor, a preset, or a record predating download_type. Those
files are written to DOWNLOAD_DIR but were linked under
audio_download/, giving a 404 on every instance where the two
directories differ.

The .mp3 clause was not an incomplete audio check to be extended with
more extensions; it was a second, conflicting rule. Removing it makes
the UI agree with where the file actually is. Same fix in
buildChapterDownloadLink, which carried a copy.

Test verified by reintroducing the bug: the video-type mp3 case fails
with 'audio_download/song.mp3' as expected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/src/app/app.spec.ts

@@ -413,4 +413,64 @@ describe('App', () => {
     expect(errorSpy).toHaveBeenCalledWith('Subscription name must not be empty');
     errorSpy.mockRestore();
   });
+  // Issue #533: the server picks AUDIO_DOWNLOAD_DIR on download_type alone
+  // (ytdl.py), so the UI's choice of URL base has to use the same rule. It used
+  // to also treat any .mp3 as audio, which pointed the link at audio_download/
+  // for files the server had written to DOWNLOAD_DIR.
+  describe('download links follow the server directory rule (#533)', () => {
+    const makeDownload = (over: Partial<Download>): Download => ({
+      id: 'vid1',
+      title: 'Test',
+      url: 'https://example.com/v',
+      download_type: 'video',
+      quality: 'best',
+      format: 'any',
+      folder: '',
+      custom_name_prefix: '',
+      playlist_item_limit: 0,
+      status: 'finished',
+      msg: '',
+      percent: 100,
+      speed: 0,
+      eta: 0,
+      filename: 'song.mp4',
+      checked: false,
+      ...over,
+    } as Download);
+
+    const appWithDirs = () => {
+      const fixture = TestBed.createComponent(App);
+      const app = fixture.componentInstance;
+      const downloads = TestBed.inject(DownloadsService) as unknown as DownloadsServiceStub;
+      downloads.configuration['PUBLIC_HOST_URL'] = 'download/';
+      downloads.configuration['PUBLIC_HOST_AUDIO_URL'] = 'audio_download/';
+      return app;
+    };
+
+    it('uses the audio base for an audio download', () => {
+      const app = appWithDirs();
+      const link = app.buildDownloadLink(makeDownload({ download_type: 'audio', filename: 'song.mp3' }));
+      expect(link).toBe('audio_download/song.mp3');
+    });
+
+    it('uses the video base for an mp3 produced by a video download', () => {
+      const app = appWithDirs();
+      const link = app.buildDownloadLink(makeDownload({ download_type: 'video', filename: 'song.mp3' }));
+      expect(link).toBe('download/song.mp3');
+    });
+
+    it('uses the video base for a video download', () => {
+      const app = appWithDirs();
+      const link = app.buildDownloadLink(makeDownload({ filename: 'clip.mp4' }));
+      expect(link).toBe('download/clip.mp4');
+    });
+
+    it('applies the same rule to chapter links', () => {
+      const app = appWithDirs();
+      const dl = makeDownload({ download_type: 'video' });
+      expect(app.buildChapterDownloadLink(dl, 'ch1.mp3')).toBe('download/ch1.mp3');
+      const audio = makeDownload({ download_type: 'audio' });
+      expect(app.buildChapterDownloadLink(audio, 'ch1.mp3')).toBe('audio_download/ch1.mp3');
+    });
+  });
 });

ui/src/app/app.ts

@@ -1284,7 +1284,12 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
 
   buildDownloadLink(download: Download) {
     let baseDir = this.downloads.configuration["PUBLIC_HOST_URL"];
-    if (download.download_type === 'audio' || download.filename.endsWith('.mp3')) {
+    // Must match the server's directory rule exactly: ytdl.py writes to
+    // AUDIO_DOWNLOAD_DIR on download_type alone. Treating any .mp3 as audio
+    // sent the link to audio_download/ for mp3s produced under a video-type
+    // download (a postprocessor, a preset, or a legacy record), which the
+    // server had written to DOWNLOAD_DIR -- a 404 whenever the two differ.
+    if (download.download_type === 'audio') {
       baseDir = this.downloads.configuration["PUBLIC_HOST_AUDIO_URL"];
     }
 
@@ -1382,7 +1387,8 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
 
   buildChapterDownloadLink(download: Download, chapterFilename: string) {
     let baseDir = this.downloads.configuration["PUBLIC_HOST_URL"];
-    if (download.download_type === 'audio' || chapterFilename.endsWith('.mp3')) {
+    // Same server-side rule as buildDownloadLink above.
+    if (download.download_type === 'audio') {
       baseDir = this.downloads.configuration["PUBLIC_HOST_AUDIO_URL"];
     }