feat: add centralized API service layer for frontend

- Create frontend/src/api/mod.rs with centralized HTTP client and error handling
- Implement api::auth module (login, logout, check_auth, get_user)
- Implement api::torrent module (add, action, delete, start, stop, set_label, set_priority)
- Implement api::setup module (get_status, setup)
- Implement api::settings module (set_global_limits)
- Implement api::push module (get_public_key, subscribe)
- Update all components to use api service layer instead of direct gloo_net calls
- Add thiserror dependency for error handling

Cargo.lock

@@ -1219,6 +1219,7 @@ dependencies = [
  "serde_json",
  "shared",
  "tailwind_fuse",
+ "thiserror 2.0.18",
  "uuid",
  "wasm-bindgen",
  "wasm-bindgen-futures",