fix(linux): M8 — header-suffix '+' button on Saved Connections group

Adds a flat list-add-symbolic button as the AdwPreferencesGroup
header_suffix, mirroring the GNOME Settings pattern (Wi-Fi,
Online Accounts, Bluetooth) where the primary affordance for the
group lives next to its title. Keyboard nav now reaches the
new-connection action without escaping the group to the headerbar,
and the '+' is the obvious entry point for users scanning for an
"add" action without seeing the bottom pill.

Bottom pill stays for visual prominence (the welcome page is still
the user's first surface, where a big suggested-action button reads
better than a corner icon).

Author: datlechin

linux/crates/app/src/ui/app.rs

@@ -1558,6 +1558,15 @@ impl App {
         let group = adw::PreferencesGroup::builder()
             .title(crate::tr!("Saved connections"))
             .build();
+        let header_new_btn = gtk::Button::builder()
+            .icon_name("list-add-symbolic")
+            .tooltip_text(crate::tr!("New connection"))
+            .valign(gtk::Align::Center)
+            .build();
+        header_new_btn.add_css_class("flat");
+        let s_header = sender.clone();
+        header_new_btn.connect_clicked(move |_| s_header.input(AppMsg::OpenConnect));
+        group.set_header_suffix(Some(&header_new_btn));
         let mut first_row: Option<adw::ActionRow> = None;
         for saved in &self.saved_connections {
             let subtitle = if saved.driver_id == "sqlite" {