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/lib/utils/api.ts

@@ -0,0 +1,47 @@
+import { toast } from '$lib/stores/toasts.js';
+import { getSocketId } from '$lib/stores/socket.js';
+
+export type ApiResult<T = any> = {
+	ok: boolean;
+	data: T;
+	status: number;
+};
+
+export async function api<T = any>(
+	url: string,
+	opts: RequestInit = {}
+): Promise<ApiResult<T>> {
+	const headers = new Headers(opts.headers);
+
+	// Auto-set JSON content type for non-FormData bodies
+	if (opts.body && !(opts.body instanceof FormData) && !headers.has('Content-Type')) {
+		headers.set('Content-Type', 'application/json');
+	}
+
+	// Pass socket ID for self-dedup
+	const sid = getSocketId();
+	if (sid) {
+		headers.set('X-Socket-ID', sid);
+	}
+
+	try {
+		const res = await fetch(url, { ...opts, headers });
+		let data: any = null;
+
+		const contentType = res.headers.get('content-type');
+		if (contentType?.includes('application/json')) {
+			data = await res.json();
+		}
+
+		if (!res.ok) {
+			const msg = data?.error || data?.message || `Request failed (${res.status})`;
+			toast.error(msg);
+			return { ok: false, data, status: res.status };
+		}
+
+		return { ok: true, data, status: res.status };
+	} catch {
+		toast.error('Network error. Please try again.');
+		return { ok: false, data: null as any, status: 0 };
+	}
+}