Implement UI/UX overhaul - cards, list, tabbed detail, context menu

Card view: 200px cards with 72px icons, .title-3 names, version+size
combined line, single priority badge, libadwaita .card class replacing
custom .app-card CSS.

List view: 48px rounded icons, .rich-list class, structured two-line
subtitle (description + version/size), single priority badge.

Detail view: restructured into ViewStack/ViewSwitcher with 4 tabs
(Overview, System, Security, Storage). 96px hero banner with gradient
background. Rows distributed logically across tabs.

Context menu: right-click (GestureClick button 3) and long-press on
cards and list rows. Menu items: Launch, Check for Updates, Scan for
Vulnerabilities, Integrate/Remove Integration, Open Containing Folder,
Copy Path. All backed by parameterized window actions.

CSS: removed custom .app-card rules (replaced by .card), added
.icon-rounded for list icons, .detail-banner gradient, and
.detail-view-switcher positioning.

src/ui/widgets.rs

@@ -33,6 +33,7 @@ pub fn status_badge_with_icon(icon_name: &str, text: &str, style_class: &str) ->
 }
 
 /// Create a badge showing integration status.
+#[allow(dead_code)]
 pub fn integration_badge(integrated: bool) -> gtk::Label {
     if integrated {
         status_badge("Integrated", "success")