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

index.html

@@ -82,17 +82,17 @@
         <div class="dashboard-stats" id="dashboard-stats">
             <div class="stat-card" onclick="showPage('inventory', 'dispensa')">
                 <span class="stat-icon">🗄️</span>
-                <span class="stat-value" id="stat-dispensa">0</span>
+                <span class="stat-value stat-loading" id="stat-dispensa">…</span>
                 <span class="stat-label" data-i18n="locations.dispensa">Dispensa</span>
             </div>
             <div class="stat-card" onclick="showPage('inventory', 'frigo')">
                 <span class="stat-icon">🧊</span>
-                <span class="stat-value" id="stat-frigo">0</span>
+                <span class="stat-value stat-loading" id="stat-frigo">…</span>
                 <span class="stat-label" data-i18n="locations.frigo">Frigo</span>
             </div>
             <div class="stat-card" onclick="showPage('inventory', 'freezer')">
                 <span class="stat-icon">❄️</span>
-                <span class="stat-value" id="stat-freezer">0</span>
+                <span class="stat-value stat-loading" id="stat-freezer">…</span>
                 <span class="stat-label" data-i18n="locations.freezer">Freezer</span>
             </div>
             <div class="stat-card" onclick="showPage('shopping')">