refactor(unsaved): extract createUnsavedWarning hook and UnsavedWarningBanner

Move the identical isDirty / beforeNavigate / discard pattern out of the
three admin detail pages (groups, tags, users) into a reusable
createUnsavedWarning() hook and a UnsavedWarningBanner presentational
component.

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

frontend/src/lib/hooks/useUnsavedWarning.svelte.ts

@@ -0,0 +1,46 @@
+import { beforeNavigate, goto } from '$app/navigation';
+
+export function createUnsavedWarning() {
+	let isDirty = $state(false);
+	let showUnsavedWarning = $state(false);
+	let discardTarget: string | null = $state(null);
+
+	beforeNavigate(({ cancel, to }) => {
+		if (isDirty) {
+			cancel();
+			showUnsavedWarning = true;
+			discardTarget = to?.url.href ?? null;
+		}
+	});
+
+	function markDirty() {
+		isDirty = true;
+		showUnsavedWarning = false;
+	}
+
+	function discard() {
+		isDirty = false;
+		showUnsavedWarning = false;
+		if (discardTarget) goto(discardTarget);
+	}
+
+	function clearOnSuccess() {
+		isDirty = false;
+		showUnsavedWarning = false;
+	}
+
+	return {
+		get isDirty() {
+			return isDirty;
+		},
+		get showUnsavedWarning() {
+			return showUnsavedWarning;
+		},
+		get discardTarget() {
+			return discardTarget;
+		},
+		markDirty,
+		discard,
+		clearOnSuccess
+	};
+}