refactor(admin-tags): migrate tag-edit page from $app/stores to $app/state

The legacy $app/stores subscription API is replaced with the modern
$app/state reactive proxy (page.url.pathname), per ADR-012's
architectural follow-on. The two spec mocks of $app/stores are replaced
with sync-factory $app/state mocks, matching the existing convention in
aktivitaeten/documents specs. Part of #560.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/admin/tags/[id]/+page.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 import { enhance } from '$app/forms';
 import { replaceState } from '$app/navigation';
-import { page } from '$app/stores';
+import { page } from '$app/state';
 import { m } from '$lib/paraglide/messages.js';
 import { createUnsavedWarning } from '$lib/shared/hooks/useUnsavedWarning.svelte';
 import UnsavedWarningBanner from '$lib/shared/primitives/UnsavedWarningBanner.svelte';
@@ -44,7 +44,7 @@ $effect(() => {
 
 $effect(() => {
 	if (data.mergeSuccess) {
-		replaceState($page.url.pathname, {});
+		replaceState(page.url.pathname, {});
 	}
 });
 

frontend/src/routes/admin/tags/[id]/page.svelte.spec.ts

@@ -10,12 +10,9 @@ vi.mock('$app/navigation', () => ({
 	goto: vi.fn(),
 	replaceState: vi.fn()
 }));
-vi.mock('$app/stores', () => ({
-	page: {
-		subscribe: (fn: (v: { url: URL }) => void) => {
-			fn({ url: new URL('http://localhost/admin/tags/t1') });
-			return () => {};
-		}
+vi.mock('$app/state', () => ({
+	get page() {
+		return { url: new URL('http://localhost/admin/tags/t1') };
 	}
 }));
 

frontend/src/routes/admin/tags/[id]/page.svelte.test.ts

@@ -2,14 +2,11 @@ import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
 
-const mockPage = { url: { pathname: '/admin/tags/t1' } };
+const mockPage = { url: new URL('http://localhost/admin/tags/t1') };
 
-vi.mock('$app/stores', () => ({
-	page: {
-		subscribe: (fn: (v: typeof mockPage) => void) => {
-			fn(mockPage);
-			return () => {};
-		}
+vi.mock('$app/state', () => ({
+	get page() {
+		return mockPage;
 	}
 }));