web-player: big pages (Explore, Home) and tracks.ts cleanup

This pass also tackles the task deferred earlier: removing
tracks.ts's TrackTarget wrapper and its 8 accessors
(targetId/targetTitle/targetArtist/targetOwnerUserId/targetExplicit/
targetListensCount/isTargetCurrent/queueItemForTarget/
targetForQueueItem). Explore, Home, Mix, menus.svelte.ts and mixes.ts
now use track objects directly, with
toQueueItem/trackFromQueueItem (new, in player.svelte.ts) as the only
converters. Along the way, TypeScript revealed that isExplicit
doesn't exist in the real /tracks schema: Explore's "explicit" badge
was already a no-op before this refactor (always undefined ??
false); the binding is removed instead of pretending support that
never existed.

Explore: GENRE_INFO/genreTiles are extracted to src/lib/genres.ts;
the search logic
(showGroup/capped/matchPlaylists/searchCounts/searchChips/findTopResult)
is extracted to src/lib/search.ts as pure functions parameterized by
filter/query instead of closing over component state. "Top result"
unifies the duplicated button/link with <svelte:element>. Decision
made without blocking: the 4 group blocks (Tracks/Albums/Users/
Playlists) are not collapsed into a single {#each groups} — each
group has genuinely different snippets, context menu and ListRow
props, and a generic config would have added more indirection than it
removed.

Home: uses Page instead of a hand-rolled page-x; the 3 stats become
an {#each}; the hero/spotlight backgrounds (inline style= with CSS
variables) become the hero-surface/spotlight-surface utilities, and
gap-11.5 is replaced by the --spacing-section token (2.875rem) via
gap-section.

Verified with npm run check (0 errors) and npm run build. /explore
without a session redirects to /login because +layout.server.ts
requires allowAnonymousListening from the backend (unreachable
without Docker this session) — pre-existing behavior, not a
regression. No authenticated visual QA; smoke-tested /login with no
console errors.

web-player/src/routes/layout.css

@@ -57,6 +57,7 @@
 	--spacing-icon-md: 1.125rem;
 	--spacing-icon-lg: 1.25rem;
 	--spacing-icon-xl: 2.25rem;
+	--spacing-section: 2.875rem;
 
 	--container-prose-sm: 26rem;
 	--container-hero: 34rem;
@@ -203,6 +204,19 @@
 		oklch(var(--mf-tile-shade-l) var(--mf-tile-shade-c) var(--tile-hue))
 	);
 }
+@utility hero-surface {
+	background-image: var(--mf-hero-bg);
+}
+@utility spotlight-surface {
+	background-image:
+		var(--mf-spotlight-bg),
+		linear-gradient(
+			to right,
+			transparent 50%,
+			color-mix(in oklch, var(--mf-scrim) 22%, transparent) 61%,
+			transparent 92.5%
+		);
+}
 @utility field-control {
 	@apply w-full rounded-control bg-surface py-3 text-sm text-fg transition placeholder:text-muted focus:bg-surface-2 focus:outline-none;
 }