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/api.rs

@@ -42,14 +42,26 @@ struct Summary {
 
 use crate::model::Article;
 
+fn normalize_base_url(server_url: &str) -> String {
+    let base = server_url.trim_end_matches('/');
+    // If the user entered just a host (or host/path) without the FreshRSS
+    // API suffix, append it automatically.
+    if base.ends_with("/api/greader.php") {
+        base.to_string()
+    } else {
+        format!("{base}/api/greader.php")
+    }
+}
+
 impl Api {
     pub async fn login(
         server_url: &str,
         username: &str,
         password: &str,
     ) -> Result<Self, String> {
+        let base = normalize_base_url(server_url);
         let client = Client::new();
-        let url = format!("{}/accounts/ClientLogin", server_url.trim_end_matches('/'));
+        let url = format!("{base}/accounts/ClientLogin");
         let resp = client
             .post(&url)
             .form(&[("Email", username), ("Passwd", password)])
@@ -72,7 +84,7 @@ impl Api {
 
         Ok(Self {
             client,
-            server_url: server_url.trim_end_matches('/').to_string(),
+            server_url: base,
             auth_token,
         })
     }