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.

README.md

@@ -54,6 +54,37 @@ links from `/admin/users`.
 The env admin token still exists as emergency fallback access. Set `WARPBOX_ADMIN_TOKEN` and use it
 at `/admin/login` if you need to recover access without a user session.
 
+## Emoji reaction packs
+
+File reactions use emoji packs from the runtime data directory, not from the application code. By
+default that means `./data/emoji`; if you change `WARPBOX_DATA_DIR`, use
+`$WARPBOX_DATA_DIR/emoji` instead.
+
+Each folder under `./data/emoji` becomes one emoji tab in the reaction picker. Put image files
+directly inside the pack folder:
+
+```text
+data/
+├── db/
+├── files/
+├── logs/
+└── emoji/
+    ├── openmoji/
+    │   ├── 1F600.svg
+    │   ├── 1F44D.svg
+    │   └── 2764.svg
+    ├── pixel-pack/
+    │   ├── happy.webp
+    │   ├── fire.webp
+    │   └── star.webp
+    └── custom-work/
+        ├── approved.png
+        └── shipped.png
+```
+
+In this example, the picker shows tabs named `Openmoji`, `Pixel pack`, and `Custom work`.
+Supported emoji image extensions are `.svg`, `.webp`, `.png`, `.jpg`, `.jpeg`, and `.gif`.
+
 For one-off Go commands, run them from the backend module:
 
 ```bash