fix: tokio runtime, Enter-to-login, and server URL handling

Three bugs fixed:

- No tokio reactor: glib::spawn_future_local does not provide a
  tokio context, so reqwest/hyper panicked at runtime. Introduce
  src/runtime.rs with a multi-thread tokio Runtime (init() called
  from main before the GTK app starts). runtime::spawn() posts the
  async result back to GTK via a tokio oneshot channel awaited by
  glib::spawn_future_local, which only polls a flag (no I/O).
  runtime::spawn_bg() is used for fire-and-forget background calls.

- Enter key didn't submit login: connect_apply on AdwEntryRow only
  fires when show-apply-button is true. Switch to connect_entry_activated
  which fires on Return in all three login rows.

- Wrong API URL: the app constructed /accounts/ClientLogin directly
  off the server host, yielding a 404. Add normalize_base_url() in
  api.rs that appends /api/greader.php when the URL doesn't already
  contain it, so users can enter just https://rss.example.com.

src/runtime.rs

@@ -0,0 +1,50 @@
+use std::sync::OnceLock;
+use tokio::runtime::Runtime;
+
+static RT: OnceLock<Runtime> = OnceLock::new();
+
+pub fn init() {
+    RT.get_or_init(|| {
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()
+            .expect("failed to build tokio runtime")
+    });
+}
+
+/// Spawn `future` on the tokio runtime. When it completes, `callback`
+/// is invoked on the GLib main context (GTK main thread).
+///
+/// Works by routing the result through a tokio oneshot channel; the
+/// receiving end is awaited by `glib::spawn_future_local`, which runs
+/// on the GLib event loop but does no I/O, so it never needs tokio's
+/// reactor itself.
+pub fn spawn<F, T, C>(future: F, callback: C)
+where
+    F: std::future::Future<Output = T> + Send + 'static,
+    T: Send + 'static,
+    C: FnOnce(T) + 'static,
+{
+    let (tx, rx) = tokio::sync::oneshot::channel::<T>();
+
+    RT.get().expect("runtime not initialised").spawn(async move {
+        let result = future.await;
+        let _ = tx.send(result);
+    });
+
+    // The receive future only polls a mutex-protected flag — no I/O,
+    // so running it on the GLib event loop is fine.
+    glib::spawn_future_local(async move {
+        if let Ok(value) = rx.await {
+            callback(value);
+        }
+    });
+}
+
+/// Fire-and-forget: spawn on the tokio runtime, no callback.
+pub fn spawn_bg<F>(future: F)
+where
+    F: std::future::Future<Output = ()> + Send + 'static,
+{
+    RT.get().expect("runtime not initialised").spawn(future);
+}