refactor: migrate all page.server.ts files to typed API client

All server-side fetch calls now go through createApiClient() from
$lib/api.server.ts, which wraps openapi-fetch with the generated OpenAPI
types. This means backend changes are reflected in the frontend after
running npm run generate:api.

- Add stub src/lib/generated/api.ts (replaced by generate:api output)
- Fix GroupController: missing /api prefix and ResponseStatusException
- Root, conversations, persons, documents pages all use typed client
- Error handling uses apiError.code directly (no parseBackendError needed)
- Edit page load uses typed client; PUT action keeps raw fetch (multipart)
- Login keeps raw fetch (explicit Authorization header, not cookie auth)

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

frontend/src/routes/persons/+page.server.ts

@@ -1,18 +1,12 @@
-import { redirect } from '@sveltejs/kit';
-import { env } from '$env/dynamic/private';
+import { createApiClient } from '$lib/api.server';
 
 export async function load({ url, fetch }) {
     const q = url.searchParams.get('q') || '';
-    const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+    const api = createApiClient(fetch);
 
-    // Query Parameter an Backend durchreichen
-    const apiUrl = new URL(`${baseUrl}/api/persons`);
-    if (q) apiUrl.searchParams.set('q', q);
+    const { data } = await api.GET('/api/persons', {
+        params: { query: { q: q || undefined } }
+    });
 
-    const res = await fetch(apiUrl.toString());
-
-    if (!res.ok) return { persons: [] };
-
-    const persons = await res.json();
-    return { persons, q };
+    return { persons: data ?? [], q };
 }