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/components/UnsavedWarningBanner.svelte

@@ -0,0 +1,22 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+interface Props {
+	onDiscard: () => void;
+}
+
+let { onDiscard }: Props = $props();
+</script>
+
+<div
+	class="mb-5 flex items-center justify-between rounded border border-amber-200 bg-amber-50 p-3 text-sm text-amber-800 dark:border-amber-800 dark:bg-amber-950/40 dark:text-amber-300"
+>
+	<span>{m.admin_unsaved_warning()}</span>
+	<button
+		type="button"
+		onclick={onDiscard}
+		class="ml-4 shrink-0 font-sans text-xs font-bold tracking-widest text-amber-800 uppercase hover:text-amber-900 dark:text-amber-300"
+	>
+		{m.person_discard_changes()}
+	</button>
+</div>