Stop image picker thumbnail flicker

The May 17 picker performance change made activated thumbnail loads asynchronous, which undid the earlier anti-flicker fix and let carousel movement flash blank thumbnails. Keep lazy activation, but load activated thumbnails synchronously so theme/background picker navigation remains stable. Add regression coverage for the thumbnail loading mode.

shell/plugins/image-picker/ImagePicker.qml

@@ -547,7 +547,7 @@ Item {
                   // selection moves through the carousel.
                   source: item.sourceActivated && item.thumbnailPath ? Util.fileUrl(item.thumbnailPath) : ""
                   fillMode: Image.PreserveAspectCrop
-                  asynchronous: true
+                  asynchronous: false
                   cache: true
                   smooth: true
                 }

test/shell.d/image-picker-test.sh

@@ -47,4 +47,8 @@ assert(
   /function preloadRows[\s\S]*if \(opened \|\| requestActive\) return/.test(imagePickerQml),
   'image picker ignores cache preloads while a request is visible'
 )
+assert(
+  /source: item\.sourceActivated && item\.thumbnailPath \? Util\.fileUrl\(item\.thumbnailPath\) : ""[\s\S]*asynchronous: false/.test(imagePickerQml),
+  'image picker loads activated thumbnails synchronously to avoid carousel flicker'
+)
 JS