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/components/chrome.test.ts

@@ -11,7 +11,8 @@ import '@components/library-filter/library-filter';
 import '@components/library-status-indicator/library-status-indicator';
 import { Events } from '../../src/events';
 import { activeViewStore } from '@store/active-view-store';
-import { emit, stub, flush, calls, lastArgs } from '@test/support/harness';
+import { libraryStore } from '@store/library-store';
+import { emit, stub, flush, calls, lastArgs, resetHarness } from '@test/support/harness';
 import {
   fixture,
   shadow,
@@ -181,6 +182,14 @@ describe('<library-filter>', () => {
   it('selects a library by id, and the merged view by empty string', async () => {
     const el = await fixture('library-filter');
 
+    // The list has to be in use for the filter change to refetch it
+    // (#280): a collection nothing has loaded is not loaded to correct
+    // it, so this is the state the app is in when the Tracks view is up.
+    stub('library.Library.GetTrackTable', { strings: [''] });
+    await libraryStore.getTracks();
+    resetHarness();
+    stub('library.Library.GetTrackTable', { strings: [''] });
+
     await flush();
     await el.updateComplete;