feat(ui): theme system — amber/slate/rose, profile picker, full token migration

- Add CSS custom property token system in app.css (@theme + [data-theme] overrides)
- Three themes: amber (default), slate (indigo/dark), rose (dark pink)
- Flash prevention via inline <script> in <svelte:head> sets data-theme before paint
- Theme context (setContext/getContext) in +layout.svelte for live preview
- Theme persisted via PocketBase user_settings (PBUserSettings.theme field)
- /api/settings GET/PUT updated to handle theme field alongside existing settings
- Profile page: new Appearance section with 3 colour-swatch theme picker
- Full token migration across all 36 route/component files:
  zinc/amber hardcoded Tailwind classes → CSS var utilities (bg-(--color-surface), etc.)
- UI primitives (Badge, Button, Card, Dialog, Separator, Textarea) migrated
- accent-amber-400 replaced with inline style accent-color: var(--color-brand)

ui/src/routes/terms/+page.svelte

@@ -3,14 +3,14 @@
 </svelte:head>
 
 <div class="max-w-2xl mx-auto py-10 px-4">
-	<h1 class="text-2xl font-bold text-zinc-100 mb-6">Terms of Service</h1>
+	<h1 class="text-2xl font-bold text-(--color-text) mb-6">Terms of Service</h1>
 
-	<div class="space-y-5 text-sm text-zinc-400 leading-relaxed">
+	<div class="space-y-5 text-sm text-(--color-muted) leading-relaxed">
 		<p>
 			By using libnovel you agree to these terms. If you do not agree, please do not use the service.
 		</p>
 
-		<h2 class="text-base font-semibold text-zinc-200 mt-6">Use of the service</h2>
+		<h2 class="text-base font-semibold text-(--color-text) mt-6">Use of the service</h2>
 		<ul class="list-disc list-inside space-y-2 pl-1">
 			<li>libnovel is provided for personal, non-commercial reading use only.</li>
 			<li>You may not scrape, crawl, or systematically download content from the site.</li>
@@ -18,34 +18,34 @@
 			<li>Accounts may be suspended or terminated for abuse.</li>
 		</ul>
 
-		<h2 class="text-base font-semibold text-zinc-200 mt-6">Content</h2>
+		<h2 class="text-base font-semibold text-(--color-text) mt-6">Content</h2>
 		<p>
 			libnovel aggregates publicly available web novel content from third-party sources for
 			personal reading convenience. We do not claim ownership of any novel content displayed on
 			the site. If you are a rights holder and wish to have content removed, please see our
-			<a href="/dmca" class="text-amber-400 hover:text-amber-300 transition-colors">DMCA policy</a>.
+			<a href="/dmca" class="text-(--color-brand) hover:text-(--color-brand-dim) transition-colors">DMCA policy</a>.
 		</p>
 
-		<h2 class="text-base font-semibold text-zinc-200 mt-6">Accounts</h2>
+		<h2 class="text-base font-semibold text-(--color-text) mt-6">Accounts</h2>
 		<p>
 			You are responsible for maintaining the security of your account. libnovel is not liable
 			for any loss or damage resulting from unauthorised access to your account.
 		</p>
 
-		<h2 class="text-base font-semibold text-zinc-200 mt-6">Disclaimer of warranties</h2>
+		<h2 class="text-base font-semibold text-(--color-text) mt-6">Disclaimer of warranties</h2>
 		<p>
 			The service is provided "as is" without warranty of any kind. We do not guarantee
 			availability, accuracy, or completeness of any content. See our full
-			<a href="/disclaimer" class="text-amber-400 hover:text-amber-300 transition-colors">disclaimer</a>
+			<a href="/disclaimer" class="text-(--color-brand) hover:text-(--color-brand-dim) transition-colors">disclaimer</a>
 			for details.
 		</p>
 
-		<h2 class="text-base font-semibold text-zinc-200 mt-6">Changes to these terms</h2>
+		<h2 class="text-base font-semibold text-(--color-text) mt-6">Changes to these terms</h2>
 		<p>
 			We may update these terms at any time. Continued use of the service after changes are
 			posted constitutes acceptance of the revised terms.
 		</p>
 
-		<p class="text-zinc-600 text-xs mt-8">Last updated: {new Date().getFullYear()}</p>
+		<p class="text-(--color-muted) text-xs mt-8">Last updated: {new Date().getFullYear()}</p>
 	</div>
 </div>