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/.gitignore

@@ -26,4 +26,5 @@ vite.config.ts.timestamp-*
 src/lib/paraglide
 
 # Generated OpenAPI types — regenerate with: npm run generate:api
-src/lib/generated/api.ts
+# (committed as a stub; overwritten by the real spec after generation)
+# src/lib/generated/api.ts