Phase 8: Polish & UX — dark mode, toasts, keyboard shortcuts, responsive, accessibility

- Add dark mode with theme toggle, localStorage/cookie persistence, anti-FOUC script
- Add toast notification system with auto-dismiss and color-coded types
- Add api() fetch wrapper with auto JSON, X-Socket-ID, and error toasts
- Add keyboard shortcuts (/ search, b boards, ? help, n new card, Esc close)
- Add error page with status code and navigation
- Add focus trap utility and apply to CardModal with full ARIA dialog support
- Add hamburger menu for mobile, responsive columns, fullscreen card modal on mobile
- Add aria-labels to icon-only buttons and DnD live region
- Migrate all ~30 fetch calls to api() wrapper with success toasts
- Add dark: Tailwind variants to all 20+ pages and components

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/+page.svelte

@@ -7,8 +7,8 @@
 </svelte:head>
 
 <div class="flex flex-col items-center justify-center px-4 py-20">
-	<h1 class="text-4xl font-bold text-gray-900 mb-4">Pounce on your tasks</h1>
-	<p class="text-lg text-gray-600 mb-8 text-center max-w-xl">
+	<h1 class="text-4xl font-bold text-gray-900 dark:text-gray-100 mb-4">Pounce on your tasks</h1>
+	<p class="text-lg text-gray-600 dark:text-gray-400 mb-8 text-center max-w-xl">
 		An open source kanban board for your team. Create boards, organize tasks, and track progress in real-time.
 	</p>
 	<div class="flex gap-3">
@@ -20,7 +20,7 @@
 		</a>
 		<a
 			href="/boards"
-			class="rounded-lg border border-gray-300 px-6 py-3 text-gray-700 font-medium hover:bg-gray-100 transition"
+			class="rounded-lg border border-gray-300 dark:border-gray-600 px-6 py-3 text-gray-700 dark:text-gray-300 font-medium hover:bg-gray-100 dark:hover:bg-gray-800 transition"
 		>
 			Browse Boards
 		</a>