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/main.rs

@@ -4,6 +4,7 @@ mod article_row;
 mod credentials;
 mod login_dialog;
 mod model;
+mod runtime;
 mod window;
 
 fn main() -> glib::ExitCode {
@@ -12,6 +13,10 @@ fn main() -> glib::ExitCode {
         std::env::set_var("GSETTINGS_SCHEMA_DIR", env!("GSETTINGS_SCHEMA_DIR"));
     }
 
+    // Start the tokio multi-thread runtime before the GTK app so that
+    // reqwest/hyper can find it when API futures are spawned.
+    runtime::init();
+
     let app = app::FeedTheMonkeyApp::new();
     app.run()
 }