feat(files): add file management UI for scenarios and runs

- add scenario upload and download UI with typed file API helpers
- show run artifacts on run detail pages after polling completes
- handle multipart uploads and absolute file paths for downloads

client/src/i18n/locales/en.json

@@ -295,5 +295,21 @@
     "action_export": "Export",
     "action_import": "Import",
     "import_error": "Failed to import snippet"
+  },
+  "files": {
+    "title": "Files",
+    "upload": "Upload",
+    "uploadTitle": "Upload File",
+    "uploadSuccess": "File uploaded successfully",
+    "name": "Name",
+    "mimeType": "MIME Type",
+    "size": "Size",
+    "expiresAt": "Expires At",
+    "expiresAtLabel": "Expires At (optional)",
+    "createdAt": "Created",
+    "download": "Download",
+    "empty": "No files uploaded.",
+    "totalCount": "Total: {{count}} file(s)",
+    "artifactsTitle": "Artifacts"
   }
 }