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/documents/[id]/+page.server.ts

@@ -1,24 +1,21 @@
 import { error, redirect } from '@sveltejs/kit';
-import { env } from '$env/dynamic/private';
-import { parseBackendError, getErrorMessage } from '$lib/errors';
+import { createApiClient } from '$lib/api.server';
+import { getErrorMessage } from '$lib/errors';
 
 export async function load({ params, fetch }) {
     const { id } = params;
-    const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+    const api = createApiClient(fetch);
 
-    try {
-        const res = await fetch(`${baseUrl}/api/documents/${id}`);
+    const { data, error: apiError, response } = await api.GET('/api/documents/{id}', {
+        params: { path: { id } }
+    });
 
-        if (res.status === 401) throw redirect(302, '/login');
+    if (response.status === 401) throw redirect(302, '/login');
 
-        if (!res.ok) {
-            const backendError = await parseBackendError(res);
-            throw error(res.status, getErrorMessage(backendError?.code));
-        }
-
-        return { document: await res.json() };
-    } catch (e) {
-        if (e.status) throw e;
-        throw error(500, getErrorMessage('INTERNAL_ERROR'));
+    if (apiError) {
+        const code = (apiError as { code?: string })?.code;
+        throw error(response.status, getErrorMessage(code));
     }
+
+    return { document: data };
 }