perf(library): load a collection when a view needs it, not at startup

All four collections were fetched at DOMContentLoaded and refetched on
every invalidation, whichever view was showing. On a 26 138-track
library the track list was 20.5 MB of that, and encoding it cost the
backend ~170 MB of transient allocation — paid by someone looking at
Home, which draws none of it.

- The store warms only albums, artists and genres, on idle after first
  paint: 1.6 MB together, and what made those views instant.
- A nav item prefetches on hover and on keyboard focus, which is the
  ~100 ms before the click that a cold open would otherwise wait.
- An invalidation refetches what something had loaded, and nothing
  else — a scan no longer loads the track list of a library whose
  Tracks view nobody has opened.
- `index.html`'s first-paint `<track-list>` is `view-hidden`, and
  `index.ts` no longer activates it: it is markup, not a decision about
  which view the launch lands on, and activating it was what fetched
  the whole list for a landing on Home. The track list itself loads on
  view activation, which the shell drives.

Measured on 50 000 tracks: backend RSS at rest 543 → 296 MB, peak 571 →
296 MB, Go heap held 361 → 125 MB, JS heap 31.8 → 18 MB, binding bytes
at rest 35.9 → 12.1 MB, heap after a browse 36.5 → 22.7 MB. Tracks
first open 26 ms; slowest view open 57 ms.

Closes #280

frontend/test/stores/library-store.test.ts

@@ -43,16 +43,25 @@ function stubReads(): void {
 }
 
 /**
- * Drop the cache and let the eager refetch settle, so each test starts
- * from the same place. The store has no reset of its own; a scan
- * completing is how the app itself clears it.
+ * Start each test from a loaded store. The store has no reset of its
+ * own; a scan completing is how the app itself clears it — and since
+ * #280 it reloads only what something had loaded, so the four reads
+ * here are what says "this test is about a loaded store".
  */
 async function reload(): Promise<void> {
   stubReads();
   emit(Events.LibraryScanComplete);
   await flush();
-  // The eager refetch the invalidation kicks off is recorded like any
-  // other call; clear it, or every count in every test is off by one.
+  resetHarness();
+  stubReads();
+  await Promise.all([
+    libraryStore.getTracks(),
+    libraryStore.getAlbums(),
+    libraryStore.getArtists(),
+    libraryStore.getGenres(),
+  ]);
+  // Those reads are recorded like any other call; clear them, or every
+  // count in every test is off by one.
   resetHarness();
   stubReads();
 }
@@ -91,7 +100,7 @@ describe('library store: caching', () => {
     ]).toEqual([['/a.mp3', '/b.mp3'], ALBUMS, ARTISTS, GENRES]);
   });
 
-  it('refetches everything when a scan completes', async () => {
+  it('refetches everything a view had loaded when a scan completes', async () => {
     emit(Events.LibraryScanComplete);
     await flush();