feat: add emoji reaction support for files

- Implement `ReactionService` to manage file reactions in the database.
- Add `POST /d/{boxID}/f/{fileID}/react` endpoint to handle user reactions.
- Add `GET /emoji/{pack}/{file}` endpoint to serve custom emoji assets.
- Support loading custom emoji packs dynamically from the data directory.
- Update README with instructions on configuring emoji reaction packs.

backend/static/css/90-responsive.css

@@ -220,6 +220,16 @@
     grid-template-columns: 1fr;
   }
 
+  .file-reaction-dock {
+    right: 0.5rem;
+    bottom: 0.45rem;
+  }
+
+  .reaction-button {
+    opacity: 1;
+    transform: none;
+  }
+
   .file-progress-side {
     width: 100%;
   }