perf(library): send the track list as a dictionary-encoded column table

GetTracks answered with one JSON object per track: 20.5 MB at 26k
tracks, ~350 bytes a row of repeated key names, four cover URLs that
are identical across an album, and the album, artist and genre strings
repeated on every track. Encoding it was ~170 MB of transient Go
allocation, and parsing it the WebView's memory peak.

GetTrackTable replaces it: one array per column, every repeated
string stored once and sent as an index, genre lists interned, and only
the fields the Tracks view reads. LastPlayed and the three larger cover
tiers are left to the details dialog, which now fetches whole tracks by
path. Each row still goes through trackFromRow, so this is an encoding
of the one projection, not a second one. 167 bytes a track against 929
in the test library; a Go test holds it under a quarter of the object
encoding, and a Vitest test reads the Go columns and the generated
Track interface and fails if they drift.

An empty library is now an empty table rather than the "no tracks in
library" error the old binding returned.

Closes #281

e2e/support/fixtures.ts

@@ -96,6 +96,36 @@ export async function callBinding<T = unknown>(
   ) as Promise<T>;
 }
 
+/** A library track as the specs use it: the path and its names. */
+export interface LibraryTrack {
+  FilePath: string;
+  TrackName: string;
+  Album: string;
+}
+
+/**
+ * Every track in the library, in the order the backend lists them.
+ *
+ * The list arrives as a columnar `TrackTable` since #281 (repeated
+ * strings sent once, as indexes into `strings`), so it cannot be read
+ * as an array of tracks any more. This reads the three columns the
+ * specs use; `frontend/src/utils/track-table.ts` is the real decoder.
+ */
+export async function libraryTracks(page: Page): Promise<LibraryTrack[]> {
+  const t = await callBinding<{
+    strings: string[];
+    filePath: string[];
+    trackName: number[];
+    album: number[];
+  }>(page, 'library.Library.GetTrackTable', [0]);
+
+  return (t.filePath ?? []).map((FilePath, i) => ({
+    FilePath,
+    TrackName: t.strings[t.trackName[i]!] ?? '',
+    Album: t.strings[t.album[i]!] ?? '',
+  }));
+}
+
 /**
  * The binding calls the *app* made, newest last, as `pkg.Type.Method`.
  *