add thumbnail preview for each queued image.

Create and revoke blob URLs on pick and session reset; reset upload session on view unmount.

frontend/src/utils/upload/revokeQueueThumbnailUrls.ts

@@ -0,0 +1,5 @@
+export const revokeQueueThumbnailUrls = (urlsByKey: Record<string, string>): void => {
+    for (const url of Object.values(urlsByKey)) {
+        URL.revokeObjectURL(url);
+    }
+};