Improve Media Manager loading state with parallel fetching and overlay

- Implemented simultaneous execution of getMediaDiagnostics and getMediaForSku in MediaManager.html to speed up initial load and refresh.
- Added a translucent grid-loading-overlay that appears over existing tiles during refresh, preventing interaction while maintaining context.
- Differentiated loading messages: 'Connecting to systems...' for initial load vs 'Refreshing media...' for updates.
- Fixed a syntax error in the save handler.

src/MediaManager.html

@@ -477,6 +477,25 @@
       padding-top: 12px;
       border-top: 1px solid #f1f5f9;
     }
+
+    /* Grid Overlay */
+    .grid-loading-overlay {
+      position: absolute;
+      top: 0;
+      left: 0;
+      right: 0;
+      bottom: 0;
+      background: rgba(255, 255, 255, 0.75);
+      backdrop-filter: blur(1px);
+      z-index: 50;
+      display: flex;
+      flex-direction: column;
+      align-items: center;
+      justify-content: center;
+      color: var(--text-secondary);
+      font-weight: 500;
+      border-radius: 8px;
+    }
   </style>
 </head>
 
@@ -560,8 +579,8 @@
           </button>
         </div>
         <input type="file" id="file-input" multiple style="display:none" onchange="controller.handleFiles(this.files)">
-<!-- Unified Transfer Session UI -->
-<div id="transfer-session-ui" class="transfer-session" style="display:none;">
+        <!-- Unified Transfer Session UI -->
+        <div id="transfer-session-ui" class="transfer-session" style="display:none;">
           <!-- Instructions (Top) -->
           <div id="transfer-desc" style="font-size:13px; color:var(--text); margin-bottom:12px; line-height:1.4;">
             <!-- Dynamic Helper Text -->
@@ -983,10 +1002,31 @@
     };
 
     UI.prototype.setLoadingState = function (isLoading) {
+      var overlay = document.getElementById('grid-loading-overlay');
+
       if (isLoading) {
+        // Check if we have items
+        var hasItems = this.grid.children.length > 0 && !this.grid.querySelector('.empty-state');
+
+        if (hasItems) {
+          // Create overlay if not exists
+          if (!overlay) {
+            overlay = document.createElement('div');
+            overlay.id = 'grid-loading-overlay';
+            overlay.className = 'grid-loading-overlay';
+            overlay.innerHTML = '<div class="spinner" style="margin-bottom: 12px;"></div><div>Refreshing media...</div>';
+            this.grid.style.position = 'relative'; // Ensure positioning context
+            this.grid.appendChild(overlay);
+          }
+        } else {
+          // Standard empty state loading
           this.grid.innerHTML = '<div style="grid-column: 1 / -1; text-align: center; padding: 40px; color: var(--text-secondary);">' +
             '<div class="spinner" style="margin-bottom: 12px;"></div>' +
-          '<div>Connecting to systems...</div></div>';
+            '<div>Loading media...</div></div>';
+        }
+      } else {
+        // Clear overlay
+        if (overlay) overlay.remove();
       }
     };
 
@@ -1281,9 +1321,9 @@
 
         ui.logStatus('init', 'Initializing access...', 'info');
 
+        // 1. Diagnostics (Parallel)
         google.script.run
-          .withSuccessHandler((diagnostics) => { // Use arrow
-
+          .withSuccessHandler((diagnostics) => {
             // Check Resumption
             if (diagnostics.activeJobId) {
               ui.logStatus('resume', 'Resuming active background job...', 'info');
@@ -1304,7 +1344,6 @@
             // Capture Token
             if (diagnostics.token) state.token = diagnostics.token;
 
-
             // Shopify Status
             if (diagnostics.shopify.status === 'ok') {
               ui.logStatus('shopify', `Shopify Product: ok (${diagnostics.shopify.mediaCount} media) (ID: ${diagnostics.shopify.id}) <a href="${diagnostics.shopify.adminUrl}" target="_blank" style="margin-left:8px;">Open Admin ↗</a>`, 'success');
@@ -1314,20 +1353,26 @@
             } else {
               ui.logStatus('shopify', `Shopify Check Failed: ${diagnostics.shopify.error}`, 'error');
             }
+          })
+          .withFailureHandler(function (err) {
+            ui.logStatus('fatal', `Diagnostics failed: ${err.message}`, 'error');
+          })
+          .getMediaDiagnostics(sku, "");
 
-            ui.logStatus('fetch', 'Fetching full media state (this may take a moment)...', 'info');
-
-            // 2. Load Full Media
+        // 2. Load Full Media (Parallel)
+        ui.logStatus('fetch', 'Fetching full media state...', 'info');
         google.script.run
           .withSuccessHandler(function (items) {
             // Normalize items
-                const normalized = items.map(i => ({
+            const normalized = items.map(function (i) {
+              return {
                 ...i,
                 id: i.id || Math.random().toString(36).substr(2, 9),
-                  status: i.source || 'drive_only', // Fix: Use source as status
+                status: i.source || 'drive_only',
                 source: i.source,
-                  _deleted: false // Init soft delete flag
-                }));
+                _deleted: false
+              };
+            });
 
             state.setItems(normalized);
 
@@ -1337,20 +1382,15 @@
             } else {
               controller.showGallery();
             }
-
           })
           .withFailureHandler(function (err) {
             ui.logStatus('fatal', `Failed to load media: ${err.message}`, 'error');
+            ui.setLoadingState(false);
           })
           .getMediaForSku(sku);
-
-          })
-          .withFailureHandler(function (err) {
-            ui.logStatus('fatal', `Diagnostics failed: ${err.message}`, 'error');
-          })
-          .getMediaDiagnostics(sku, "");
       },
 
+
       saveChanges() {
         ui.toggleSave(false);
         ui.saveBtn.innerText = "Saving...";