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

frontend/src/components/layout/sidebar.rs

@@ -1,5 +1,6 @@
 use leptos::wasm_bindgen::JsCast;
 use leptos::*;
+use crate::api;
 
 #[component]
 pub fn Sidebar() -> impl IntoView {
@@ -76,12 +77,8 @@ pub fn Sidebar() -> impl IntoView {
 
     let handle_logout = move |_| {
         spawn_local(async move {
-            let client = gloo_net::http::Request::post("/api/auth/logout");
-            if let Ok(resp) = client.send().await {
-                if resp.ok() {
-                    // Force full reload to clear state
-                    let _ = window().location().set_href("/login");
-                }
+            if api::auth::logout().await.is_ok() {
+                let _ = window().location().set_href("/login");
             }
         });
     };