feat(web): a playlist entry whose file is missing greys out and is skipped — #2527

The row treatment for a dead playlist entry already existed -- muted
text, no play on click, no drag, no kebab, never "now playing" -- but it
only fired for track_id === null, the track-deleted case. A missing file
kept a live-looking row that failed on click.

The behavioural gate now covers both, and the presentation distinguishes
them, because they mean different things to the person reading the list.
A removed track is gone for good and keeps the strikethrough. A missing
file is a track we still have -- history, likes, the lot -- whose bytes
aren't on disk right now, so it gets an explicit "File missing" and a
title explaining it stays in the playlist and comes back on its own if
the file does. A strikethrough there would claim it was deleted, which
is a lie about a file the scanner may well adopt back tomorrow (#2528).

Skipping routes through playlistTrackToRef, which already returned null
for removed tracks and whose callers already filter nulls. Adding the
unavailable check there means every queue builder -- PlaylistCard,
systemRefetch, the detail page -- skips a missing file without any of
them learning what missing_since is.

Remove stays available on a dead row: the owner must still be able to
take it out of their own list.

web/src/lib/components/PlaylistTrackRow.test.ts

@@ -41,7 +41,8 @@ const live: PlaylistTrack = {
   album_title: 'MHTRTC',
   duration_sec: 137,
   stream_url: '/api/tracks/t-1/stream',
-  added_at: ''
+  added_at: '',
+  unavailable: false
 };
 
 const removed: PlaylistTrack = {
@@ -52,6 +53,15 @@ const removed: PlaylistTrack = {
   stream_url: null
 };
 
+// Missing file (#2527): the track row still exists — ids intact, history
+// intact — but the server withholds the stream URL because the bytes are
+// not on disk. Deliberately distinct from `removed` above.
+const missingFile: PlaylistTrack = {
+  ...live,
+  stream_url: null,
+  unavailable: true
+};
+
 afterEach(() => vi.clearAllMocks());
 
 describe('PlaylistTrackRow', () => {
@@ -119,4 +129,31 @@ describe('PlaylistTrackRow', () => {
     await fireEvent.click(screen.getByText('Roygbiv'));
     expect(onPlay).toHaveBeenCalledWith(2);
   });
+
+  // A missing file is NOT a removed track: the row says so, and says it
+  // without the strikethrough that would imply the track is gone for good.
+  test('missing file is labelled and not struck through', () => {
+    render(PlaylistTrackRow, {
+      props: { row: missingFile, isOwner: true, onRemove: vi.fn(), onPlay: vi.fn() }
+    });
+    expect(screen.getByText(/file missing/i)).toBeTruthy();
+    expect(screen.getByText('Roygbiv').className).not.toContain('line-through');
+  });
+
+  test('clicking a missing-file row does not play it', async () => {
+    const onPlay = vi.fn();
+    render(PlaylistTrackRow, {
+      props: { row: missingFile, isOwner: true, onRemove: vi.fn(), onPlay }
+    });
+    await fireEvent.click(screen.getByText('Roygbiv'));
+    expect(onPlay).not.toHaveBeenCalled();
+  });
+
+  // The owner must still be able to take the dead entry out of their list.
+  test('remove stays available on a missing-file row', () => {
+    render(PlaylistTrackRow, {
+      props: { row: missingFile, isOwner: true, onRemove: vi.fn(), onPlay: vi.fn() }
+    });
+    expect(screen.getByLabelText(/remove roygbiv from playlist/i)).toBeTruthy();
+  });
 });