fix(ui/chat): harden emoji favorites with size limits and mutex fix

- Add file size limit (4 KiB) when loading emoji favorites (SEC #14)
- Validate individual emoji string length (64 bytes max)
- Fix mutex ordering: save() no longer self-locks, toggle() holds
  write lock through entire operation including save (SEC #15, COMP #5)
- Add file-level doc comment (COMP #12)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/ui/chat/emoji_favorites.go

@@ -1,3 +1,5 @@
+// emoji_favorites.go handles persistence of the user's favorite emoji list,
+// stored as a JSON file in the cache directory.
 package chat
 
 import (
@@ -12,6 +14,12 @@ import (
 
 const maxFavoriteEmoji = 10
 
+// maxFavoritesFileSize is the upper bound on the emoji favorites JSON file (4 KiB).
+const maxFavoritesFileSize = 4096
+
+// maxFavoriteEmojiLen is the maximum byte length of a single emoji favorite string.
+const maxFavoriteEmojiLen = 64
+
 type emojiFavorites struct {
 	Favorites []string `json:"favorites"`
 	mu        sync.RWMutex
@@ -25,35 +33,46 @@ func loadEmojiFavorites() *emojiFavorites {
 	if err != nil {
 		return ef
 	}
+	if len(data) > maxFavoritesFileSize {
+		slog.Warn("emoji favorites file too large, ignoring", "size", len(data))
+		return ef
+	}
 	if err := json.Unmarshal(data, ef); err != nil {
 		slog.Warn("failed to parse emoji favorites", "err", err)
 		return &emojiFavorites{}
 	}
+	// Reject any favorite strings that exceed the length limit.
+	valid := ef.Favorites[:0]
+	for _, e := range ef.Favorites {
+		if len(e) <= maxFavoriteEmojiLen {
+			valid = append(valid, e)
+		} else {
+			slog.Warn("emoji favorite too long, skipping", "len", len(e))
+		}
+	}
+	ef.Favorites = valid
 	return ef
 }
 
+// save persists favorites to disk. The caller must hold ef.mu.
 func (ef *emojiFavorites) save() {
-	ef.mu.RLock()
-	defer ef.mu.RUnlock()
 	atomicSaveJSON(emojiFavoritesPath, ef)
 }
 
 func (ef *emojiFavorites) toggle(emoji string) {
 	ef.mu.Lock()
+	defer ef.mu.Unlock()
 	for i, e := range ef.Favorites {
 		if e == emoji {
 			ef.Favorites = append(ef.Favorites[:i], ef.Favorites[i+1:]...)
-			ef.mu.Unlock()
 			ef.save()
 			return
 		}
 	}
 	if len(ef.Favorites) >= maxFavoriteEmoji {
-		ef.mu.Unlock()
 		return
 	}
 	ef.Favorites = append(ef.Favorites, emoji)
-	ef.mu.Unlock()
 	ef.save()
 }