fix: improve background settings UX and fix pointer events (#598)

- add pointer-events: none to body::before (fixes input blocking)
- remove disabled state from background URL input (can set fallback)
- fix toast message when toggling playing artwork off
- update descriptions to clarify fallback behavior
- always show tile checkbox when background URL is set

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

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

frontend/src/routes/+layout.svelte

···
 		filter: blur(var(--bg-blur, 0px));
 		transform: scale(1.1); /* prevent blur edge artifacts */
 		z-index: -1;
+		pointer-events: none;
 	}
 
 	.app-layout {

frontend/src/routes/settings/+page.svelte

···
 
 	async function saveUsePlayingArtwork(enabled: boolean) {
 		await preferences.updateUiSettings({ use_playing_artwork_as_background: enabled });
-		toast.success(enabled ? 'using playing artwork as background' : 'using custom background');
+		toast.success(enabled ? 'playing artwork background enabled' : 'playing artwork background disabled');
 	}
 
 	function selectTheme(theme: Theme) {
···
 				<div class="setting-row">
 					<div class="setting-info">
 						<h3>background image</h3>
-						<p>set a custom background image (URL)</p>
+						<p>set a custom background image (URL){#if usePlayingArtwork} — used as fallback when nothing is playing{/if}</p>
 					</div>
 					<div class="background-controls">
 						<input
···
 							bind:value={backgroundInput}
 							onblur={saveBackgroundImage}
 							onkeydown={(e) => e.key === 'Enter' && saveBackgroundImage()}
-							disabled={usePlayingArtwork}
 						/>
-						{#if backgroundImageUrl && !usePlayingArtwork}
+						{#if backgroundImageUrl}
 							<label class="tile-toggle">
 								<input
 									type="checkbox"
···
 				<div class="setting-row">
 					<div class="setting-info">
 						<h3>playing artwork as background</h3>
-						<p>use the currently playing track's artwork as background (overrides custom image)</p>
+						<p>use currently playing track's artwork as background (falls back to custom image when nothing is playing)</p>
 					</div>
 					<label class="toggle-switch">
 						<input