fix: APK install conflict (PackageInstaller) + dashboard stat skeleton

APK install conflict:
- Replace ACTION_VIEW-based install with PackageInstaller.Session API (API 21+)
- PackageInstaller gives us the actual install status via BroadcastReceiver:
  STATUS_PENDING_USER_ACTION → launch system confirmation dialog automatically
  STATUS_SUCCESS → success toast
  STATUS_FAILURE_CONFLICT/INCOMPATIBLE → show AlertDialog offering to
    uninstall the old version (ACTION_DELETE) so user can re-download and install
- FileProvider no longer needed for install (still kept for other uses)
- Kiosk: derive target package from filename (gateway vs kiosk self-update)

Dashboard 0-flash:
- Replace hardcoded 0 in HTML stat-value spans with ... placeholder
- Add .stat-loading CSS class: shimmer skeleton animation (gradient sweep)
- showPage(dashboard): set ... + stat-loading before API call
- loadDashboard: remove stat-loading class and set real count after data arrives

assets/css/style.css

@@ -372,6 +372,21 @@ body {
     color: var(--primary);
 }
 
+/* Skeleton pulse while stat is loading */
+.stat-value.stat-loading {
+    color: transparent;
+    background: linear-gradient(90deg, var(--border) 25%, var(--bg-dark, #e2e8f0) 50%, var(--border) 75%);
+    background-size: 200% 100%;
+    animation: stat-shimmer 1.2s infinite;
+    border-radius: 6px;
+    min-width: 2rem;
+    display: inline-block;
+}
+@keyframes stat-shimmer {
+    0%   { background-position: 200% 0; }
+    100% { background-position: -200% 0; }
+}
+
 .stat-label {
     font-size: 0.85rem;
     color: var(--text-light);