feat: add dashboard-scoped error boundary

Closes #1195.

Adds `surfsense_web/app/dashboard/error.tsx` so unhandled errors in any
dashboard route render inside the dashboard layout (sidebar/nav stay
visible) instead of blowing out to the root-level error page. `reset()`
now retries just the failed segment.

Follows the same PostHog capture pattern as the root error.tsx and adds
a secondary "Go to dashboard home" link for recovery when `reset()`
doesn't help.

surfsense_web/app/dashboard/error.tsx

@@ -0,0 +1,44 @@
+"use client";
+
+import Link from "next/link";
+import { useEffect } from "react";
+
+export default function DashboardError({
+	error,
+	reset,
+}: {
+	error: globalThis.Error & { digest?: string };
+	reset: () => void;
+}) {
+	useEffect(() => {
+		import("posthog-js")
+			.then(({ default: posthog }) => {
+				posthog.captureException(error);
+			})
+			.catch(() => {});
+	}, [error]);
+
+	return (
+		<div className="flex flex-1 flex-col items-center justify-center gap-4 p-8 text-center">
+			<h2 className="text-xl font-semibold">Something went wrong</h2>
+			<p className="text-muted-foreground max-w-md">
+				An error occurred in this section. Your dashboard is still available.
+			</p>
+			<div className="flex gap-2">
+				<button
+					type="button"
+					onClick={reset}
+					className="rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground hover:bg-primary/90 transition-colors"
+				>
+					Try again
+				</button>
+				<Link
+					href="/dashboard"
+					className="rounded-md border border-input bg-background px-4 py-2 text-sm font-medium hover:bg-accent hover:text-accent-foreground transition-colors"
+				>
+					Go to dashboard home
+				</Link>
+			</div>
+		</div>
+	);
+}