feat(gui): branded app icon in the sidebar header and login

The sidebar header and the login page used the generic folder-remote-symbolic
icon. Ship the app icon from packaging/ in the GUI resources and use it in
both places, so the in-app brand matches the launcher even when the app runs
uninstalled.

crates/pdfs-gui/src/app/main.rs

@@ -884,8 +884,8 @@ fn build_sidebar(footer: &gtk4::Box) -> (adw::NavigationPage, gtk4::ListBox) {
     });
 
     let brand = gtk4::Box::new(gtk4::Orientation::Horizontal, 8);
-    let icon = gtk4::Image::from_icon_name("folder-remote-symbolic");
-    icon.add_css_class("brand-icon");
+    let icon = gtk4::Image::from_icon_name(APP_ID);
+    icon.set_pixel_size(24);
     brand.append(&icon);
     let name = gtk4::Label::new(Some("Proton Drive"));
     name.add_css_class("heading");