feat(mvp): phase 7 - UI polish & ambient backgrounds

Add layout system (sidebar, header, main layout), dark/light/system theme
with HSL customization, 3 ambient backgrounds (mesh gradient, particle field,
aurora), Cmd/Ctrl+K search dialog, page transitions, card hover effects,
status pulse animations, skeleton loaders, and responsive design. Polish
all existing pages with consistent theming.

src/lib/components/section/Section.svelte

@@ -38,7 +38,7 @@
 	let expanded = $state(section.isExpandedByDefault);
 </script>
 
-<div class="rounded-lg border border-gray-700 bg-gray-800/30">
+<div class="rounded-xl border border-border bg-card/30 shadow-sm">
 	<SectionHeader
 		title={section.title}
 		icon={section.icon}