fix: improve example image upload reliability and error handling, #804

- Sequential per-file upload to avoid client_max_size limits
- Add backend exception handler with proper 500 responses
- Increase standalone server upload limit to 256MB
- Add partial success localization support

locales/en.json

@@ -1466,6 +1466,7 @@
             "folderTreeFailed": "Failed to load folder tree",
             "folderTreeError": "Error loading folder tree",
             "imagesImported": "Example images imported successfully",
+            "imagesPartial": "{success} image(s) imported, {failed} failed",
             "importFailed": "Failed to import example images: {message}"
         },
         "triggerWords": {