Rewrite detail view copy for beginners, add tab transitions and lightbox fixes

- Replace technical jargon with plain language across all 4 detail tabs
- Friendly titles/subtitles with technical details available in tooltips
- Soften terminal commands ("Install with one command" instead of raw commands)
- Rename sections: Desktop Integration -> App Menu, Sandboxing -> App Isolation,
  Vulnerability Scanning -> Security Check, Capabilities -> Features
- Rewrite Wayland/FUSE explanations to avoid acronyms and dev terminology
- Update security report page with beginner-friendly descriptions
- Enable libadwaita 1.7 ViewStack crossfade transitions between detail tabs
- Rewrite screenshot lightbox as separate gtk::Window (fixes scroll jump on
  close, adds click-outside-to-close, rounded corners via CSS)
- Add prev/next navigation arrows and keyboard support to lightbox

data/resources/style.css

@@ -221,3 +221,22 @@ button.flat:not(.pill):not(.suggested-action):not(.destructive-action) {
     min-width: 24px;
     min-height: 24px;
 }
+
+/* ===== Screenshot Lightbox ===== */
+window.lightbox {
+    background-color: rgba(0, 0, 0, 0.92);
+    border-radius: 12px;
+}
+
+window.lightbox .lightbox-counter {
+    background: rgba(0, 0, 0, 0.6);
+    color: white;
+    border-radius: 12px;
+    padding: 4px 12px;
+    font-size: 0.85em;
+}
+
+window.lightbox .lightbox-nav {
+    min-width: 48px;
+    min-height: 48px;
+}