fix(ui): replace full-page loading overlay with grid-scoped loader to eliminate flicker

- Add .grid-loading-overlay CSS: position:absolute inside card grid,
  semi-transparent dark background, z-index 100, pointer-events:none
- Add showGridLoading() / hideGridLoading() to VirtualScroller:
  creates/removes the scoped overlay inside the card grid only
- Modify loadMoreWithVirtualScroll(): replace full-page
  state.loadingManager overlay with grid-scoped loading, defer
  hide via requestAnimationFrame to eliminate blank-frame gap
- Clean up gridLoadingOverlay in dispose() to prevent DOM leak

static/js/utils/VirtualScroller.js

@@ -657,6 +657,9 @@ export class VirtualScroller {
             this.resizeObserver.disconnect();
         }
 
+        // Remove any active grid loading overlay
+        this.hideGridLoading();
+
         // Remove rendered elements
         this.clearRenderedItems();
 
@@ -1130,4 +1133,30 @@ export class VirtualScroller {
             index: targetIndex
         };
     }
+
+    /**
+     * Show a grid-scoped loading indicator (replaces full-page overlay)
+     * Only covers the card grid area, leaving header/sidebar unaffected.
+     */
+    showGridLoading() {
+        // Remove any stale overlay from a prior deferred hide (e.g. from final rAF)
+        this.hideGridLoading();
+        const overlay = document.createElement('div');
+        overlay.className = 'grid-loading-overlay';
+        const spinner = document.createElement('div');
+        spinner.className = 'loading-spinner';
+        overlay.appendChild(spinner);
+        this.gridElement.appendChild(overlay);
+        this.gridLoadingOverlay = overlay;
+    }
+
+    /**
+     * Hide the grid-scoped loading indicator.
+     */
+    hideGridLoading() {
+        if (this.gridLoadingOverlay) {
+            this.gridLoadingOverlay.remove();
+            this.gridLoadingOverlay = null;
+        }
+    }
 }