Add upload queue UI with ExifTool metadata preview on pick.

Users can select images within configured limits, see per-row load status, and inspect raw ExifTool output before upload work continues.

frontend/src/workers/exiftoolWorker.ts

@@ -93,7 +93,7 @@ const processJob = async (job: ExiftoolWorkerJob): Promise<void> => {
         } catch (error) {
             self.postMessage({
                 type: 'init-error',
-                message: resolveErrorMessage(error, 'Failed to prepare ExifTool')
+                message: resolveErrorMessage(error, 'Could not prepare ExifTool')
             });
         }
 
@@ -117,7 +117,7 @@ const processJob = async (job: ExiftoolWorkerJob): Promise<void> => {
             self.postMessage({
                 type: 'read-metadata-error',
                 id: job.id,
-                message: resolveErrorMessage(error, 'Could not read image metadata')
+                message: resolveErrorMessage(error, 'Could not load image metadata')
             });
         }
     }