fix(web): resolve ReferenceError in single-file upload handler (#313)

The finally block in handleSingleFileUpload referenced an undefined
fileInput variable left over from an earlier refactor, causing an
"Unhandled promise rejection: ReferenceError" after every single-file
upload (e.g. OAuth credentials.json for the calendar plugin) even when
the upload itself succeeded.

Resolve the file input by id inside the finally block so it can be
cleared when present, tolerating the drop-zone-only case.

Co-authored-by: ChuckBuilds <ChuckBuilds@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web_interface/static/v3/js/widgets/file-upload.js

@@ -221,6 +221,7 @@
             }
             notifyFn(`Upload error: ${error.message}`, 'error');
         } finally {
+            const fileInput = document.getElementById(`${fieldId}_file_input`);
             if (fileInput) fileInput.value = '';
         }
     };