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.

data/ui/login_dialog.blp

@@ -20,6 +20,7 @@ template $LoginDialog : Adw.Dialog {
           title: _("Server URL");
           input-hints: no_spellcheck;
           input-purpose: url;
+          // e.g. https://rss.example.com — /api/greader.php is added automatically
         }
 
         Adw.EntryRow username_row {