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/js/app.js

@@ -2227,7 +2227,14 @@ function showPage(pageId, param = null) {
     
     // Page-specific init
     switch(pageId) {
-        case 'dashboard': loadDashboard(); break;
+        case 'dashboard':
+            // Show skeleton on stat-cards while data loads
+            ['dispensa', 'frigo', 'freezer'].forEach(loc => {
+                const el = document.getElementById(`stat-${loc}`);
+                if (el) { el.textContent = '…'; el.classList.add('stat-loading'); }
+            });
+            loadDashboard();
+            break;
         case 'inventory':
             if (param !== null) {
                 currentLocation = param;
@@ -2643,7 +2650,9 @@ async function loadDashboard() {
         ['dispensa', 'frigo', 'freezer'].forEach(loc => {
             const s = summary.find(x => x.location === loc);
             const count = s ? s.product_count : 0;
-            document.getElementById(`stat-${loc}`).textContent = count;
+            const el = document.getElementById(`stat-${loc}`);
+            el.textContent = count;
+            el.classList.remove('stat-loading');
             total += count;
         });
         // Add non-standard locations