feat(web): full-screen NowPlaying view

Mirrors Android's NowPlayingScreen. New /now-playing route renders
outside the Shell (no top bar, no PlayerBar) so it's a focused full
viewport: large square cover, title + linked artist/album, full-width
scrubber with timestamps, prominent transport (prev / play-pause /
next, with a 56dp circular play button), shuffle + repeat toggles,
like button, volume slider, and a queue button that opens the
existing queue drawer.

Tapping the cover or title area in PlayerBar (compact + desktop)
now navigates to /now-playing instead of the album page. Pattern
matches Spotify / YouTube Music. The album link stays reachable via
the album-name link inside NowPlaying itself, so no nav is lost.

Back button in the NowPlaying header uses history.back() with a
fallback to /. Empty state when no track is loaded points the user
at Home or the Library.

Scribe 528, local task #62.

web/src/routes/+layout.svelte

@@ -151,7 +151,7 @@
 <QueueDrawer />
 
 <QueryClientProvider client={queryClient}>
-  {#if user.value !== null && page.url.pathname !== '/login'}
+  {#if user.value !== null && page.url.pathname !== '/login' && page.url.pathname !== '/now-playing'}
     <Shell>{@render children()}</Shell>
   {:else}
     {@render children()}