fix: play button toggle works after pause on playlist/album pages (#668)

the play button on playlist and album pages wasn't resuming playback
after pausing. the issue was in the `isPlaylistPlaying` derived state
which included `!player.paused` - when paused, this became false,
causing the button to call `playNow()` instead of `togglePlayPause()`.

since the queue already had the same track at index 0, `setQueue` did
nothing and playback didn't resume.

fix: separate "is this playlist/album active" (track is from here) from
"is it playing" (active AND not paused). button now uses `isActive` to
decide toggle vs start fresh, while `isPlaying` controls the visual state.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[e-mail]>

frontend/src/routes/playlist/[id]/+page.svelte

···
 	// check if user owns this playlist
 	const isOwner = $derived(auth.user?.did === playlist.owner_did);
 
-	// check if this playlist is currently playing
-	const isPlaylistPlaying = $derived(
+	// check if current track is from this playlist (active, regardless of paused state)
+	const isPlaylistActive = $derived(
 		player.currentTrack !== null &&
-		!player.paused &&
 		tracks.some(t => t.id === player.currentTrack?.id)
 	);
+
+	// check if actively playing (not paused)
+	const isPlaylistPlaying = $derived(isPlaylistActive && !player.paused);
 </script>
 
 <svelte:window on:keydown={handleKeydown} />
···
 			<button
 				class="play-button"
 				class:is-playing={isPlaylistPlaying}
-				onclick={() => isPlaylistPlaying ? player.togglePlayPause() : playNow()}
+				onclick={() => isPlaylistActive ? player.togglePlayPause() : playNow()}
 			>
 				{#if isPlaylistPlaying}
 					<svg width="20" height="20" viewBox="0 0 24 24" fill="currentColor">

frontend/src/routes/u/[handle]/album/[slug]/+page.svelte

···
 	// can only reorder if owner and album has an ATProto list
 	const canReorder = $derived(isOwner && !!albumMetadata.list_uri);
 
-	// check if this album is currently playing
-	const isAlbumPlaying = $derived(
+	// check if current track is from this album (active, regardless of paused state)
+	const isAlbumActive = $derived(
 		player.currentTrack !== null &&
-		!player.paused &&
 		tracks.some(t => t.id === player.currentTrack?.id)
 	);
+
+	// check if actively playing (not paused)
+	const isAlbumPlaying = $derived(isAlbumActive && !player.paused);
 
 	// edit mode state
 	let isEditMode = $state(false);
···
 			<button
 				class="play-button"
 				class:is-playing={isAlbumPlaying}
-				onclick={() => isAlbumPlaying ? player.togglePlayPause() : playNow()}
+				onclick={() => isAlbumActive ? player.togglePlayPause() : playNow()}
 			>
 				{#if isAlbumPlaying}
 					<svg width="20" height="20" viewBox="0 0 24 24" fill="currentColor">