feat(admin): phase 8 — unsaved-changes guard on all detail panels

Add beforeNavigate + isDirty tracking to users/[id], users/new,
groups/[id], groups/new, and tags/[id] edit panels. When a user
navigates away with unsaved changes, the navigation is cancelled and
an inline amber warning banner appears with a Discard button that
resumes navigation. Saving successfully clears the dirty flag.

Add i18n key admin_unsaved_warning (de/en/es).
Add spec files for groups/[id] and tags/[id] panels.

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

frontend/src/routes/admin/users/new/+page.svelte

@@ -1,11 +1,24 @@
 <script lang="ts">
 import { enhance } from '$app/forms';
+import { beforeNavigate, goto } from '$app/navigation';
 import { m } from '$lib/paraglide/messages.js';
 import UserProfileSection from '$lib/components/user/UserProfileSection.svelte';
 import UserGroupsSection from '$lib/components/user/UserGroupsSection.svelte';
 import AccountSection from './AccountSection.svelte';
 
 let { data, form } = $props();
+
+let isDirty = $state(false);
+let showUnsavedWarning = $state(false);
+let discardTarget = $state<string | null>(null);
+
+beforeNavigate(({ cancel, to }) => {
+	if (isDirty) {
+		cancel();
+		showUnsavedWarning = true;
+		discardTarget = to?.url.href ?? null;
+	}
+});
 </script>
 
 <div class="flex flex-1 flex-col overflow-hidden">
@@ -16,13 +29,40 @@ let { data, form } = $props();
 
 	<!-- Scrollable body -->
 	<div class="flex-1 overflow-y-auto px-5 py-5">
+		{#if showUnsavedWarning}
+			<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={() => {
+						isDirty = false;
+						showUnsavedWarning = false;
+						if (discardTarget) goto(discardTarget);
+					}}
+					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>
+		{/if}
 		{#if form?.error}
 			<div class="mb-5 rounded border border-red-200 bg-red-50 p-3 text-sm text-red-700">
 				{form.error}
 			</div>
 		{/if}
 
-		<form id="new-user-form" method="POST" use:enhance class="space-y-5">
+		<form
+			id="new-user-form"
+			method="POST"
+			use:enhance
+			oninput={() => {
+				isDirty = true;
+				showUnsavedWarning = false;
+			}}
+			class="space-y-5"
+		>
 			<div class="rounded-sm border border-line bg-surface p-5 shadow-sm">
 				<AccountSection />
 			</div>