fix(web): a11y + reactivity warnings (paying down accumulated warnings)

Cleans up the 8 svelte-check warnings I deferred in prior CI runs.
Two distinct categories — handling them differently:

REAL REACTIVITY BUGS — fixed:
- AlbumMenu.svelte / ArtistMenu.svelte: cachedTracks initialized
  from the `tracks` prop only at component creation. When the menu
  is reused across rows (operator opens menu on one album, then
  another), the cache from the first open serves stale tracks.
  Fix: initialize cachedTracks to null; ensureTracks() reads the
  current `tracks` prop on each call before falling through to
  the network fetch.

INTENTIONAL PATTERNS LINT FLAGGED — suppressed with explanatory
comments:
- AlbumMenu / ArtistMenu / PlaylistCard menu containers: the
  onclick={(e) => e.stopPropagation()} is a guard against the
  window-level "any click closes the menu" listener — not
  user-facing interaction. A paired keyboard handler is
  intentionally absent (Escape-to-close lives on svelte:window
  onkeydown). Suppressed a11y_click_events_have_key_events with
  explanatory comments.
- CompactTrackCard overlay div: catches clicks on its inner
  LikeButton + queue button so they don't bubble to the wrapping
  play-card button. Inner buttons carry their own keyboard
  handling. Suppressed both a11y_click_events_have_key_events and
  a11y_no_static_element_interactions.
- theme.svelte.ts module-scope `_resolved = $state(resolve(_theme))`:
  module-init reads _theme to seed _resolved; subsequent updates
  go through setTheme() and the matchMedia listener. $derived
  doesn't apply at module scope. Suppressed state_referenced_locally
  with an explanatory comment.

Promised in the prior fix-forward: "I'll fix bugs when I see them
or open a follow-up task with a specific name, not park them in an
umbrella." Following through.

web/src/lib/stores/theme.svelte.ts

@@ -28,6 +28,12 @@ function applyToDOM(resolved: ResolvedTheme): void {
 }
 
 let _theme = $state<ThemePreference>(readPref());
+// svelte-ignore state_referenced_locally
+// Module-scope state with manual sync — _theme is read once at module
+// init to seed _resolved; subsequent updates flow through setTheme()
+// (which writes both) and the matchMedia listener (which writes
+// _resolved when prefers-color-scheme flips). Not a missed-reactivity
+// case; $derived doesn't apply at module scope here.
 let _resolved = $state<ResolvedTheme>(resolve(_theme));
 
 export const theme = {