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/libs/handlers/static.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"net/http"
+	"os"
 	"path/filepath"
 	"strings"
 )
@@ -15,6 +16,24 @@ func (a *App) Static() http.Handler {
 	})
 }
 
+func (a *App) EmojiAsset(w http.ResponseWriter, r *http.Request) {
+	pack := strings.TrimSpace(r.PathValue("pack"))
+	file := strings.TrimSpace(r.PathValue("file"))
+	if pack == "" || file == "" || strings.Contains(pack, "/") || strings.Contains(pack, "\\") || strings.Contains(pack, "..") || strings.Contains(file, "/") || strings.Contains(file, "\\") || strings.Contains(file, "..") || !isEmojiFile(file) {
+		http.NotFound(w, r)
+		return
+	}
+
+	path := filepath.Join(a.emojiRoot(), pack, file)
+	info, err := os.Stat(path)
+	if err != nil || info.IsDir() {
+		http.NotFound(w, r)
+		return
+	}
+	setStaticCacheHeaders(w, r.URL.Path)
+	http.ServeFile(w, r, path)
+}
+
 func setStaticCacheHeaders(w http.ResponseWriter, path string) {
 	ext := strings.ToLower(filepath.Ext(path))