fix: allow clearing display name in profile settings

- Remove restriction that prevented setting display_name to empty
- Users can now clear display name to fall back to username display
- Trim whitespace from display_name input

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

handlers/profile.go

@@ -72,9 +72,9 @@ func UpdateProfile(db *gorm.DB, storagePath string) gin.HandlerFunc {
 		}
 
 		// --- Text fields ---
-		if v := c.PostForm("display_name"); v != "" {
-			user.DisplayName = v
-		}
+		// Allow empty display_name (user can clear it to fall back to username)
+		user.DisplayName = strings.TrimSpace(c.PostForm("display_name"))
+
 		if v := c.PostForm("gender"); v != "" {
 			user.Gender = v
 		}