fix: z-index for menus/tooltips, sidebar dropdown, hi-res SVG icon

- Fix z-index across all menus and tooltips (10000+) to prevent overlap issues
- Redesign sidebar playlist header with dropdown menu: Sort by, Refresh, Import, Manage
- Add playlist tooltip on hover showing track count (lazy-loads artists)
- Fix submenu navigation with delayed close for better UX
- Replace hi-res PNG with SVG icon in QualityBadge
- Fix scrollbar spacing (8px from window edge) in main content
- Fix Svelte state_unsafe_mutation error in sidebar tooltip loading

src/lib/components/QualityBadge.svelte

@@ -77,7 +77,7 @@
 
   // Get icon path based on tier
   const iconPath = $derived.by(() => {
-    if (tier === 'max' || tier === 'hires') return '/logo-hires.png';
+    if (tier === 'max' || tier === 'hires') return '/hi-res.svg';
     if (tier === 'cd') return '/cd.svg';
     if (tier === 'lossy') return '/mp3.svg';
     return '/cd.svg';