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/lib/generated/api.ts

@@ -0,0 +1,15 @@
+/**
+ * STUB — generated by openapi-typescript
+ *
+ * Run `npm run generate:api` with the backend running in dev mode to
+ * replace this file with fully-typed definitions.
+ *
+ * While this stub is in place the typed client still works; TypeScript
+ * will start enforcing path and parameter correctness once the real
+ * types are generated.
+ */
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export type paths = Record<string, any>;
+export type components = Record<string, never>;
+export type operations = Record<string, never>;