webui: Add user birthday settings

- Store birth month and day in user_pref with default 10/18
- Migrate existing user_pref rows to fill missing birthday fields
- Expose birthday editing in the webui profile page
- Replace hardcoded birthday values in user info, top info, and product list responses
- Fix birthday month/day field alignment in the profile form

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/user/userinfo.go

@@ -18,8 +18,8 @@ func userInfo(ctx *gin.Context) {
 		ResponseData: userschema.UserInfoData{
 			User: ss.GetUserInfo(),
 			Birth: userschema.Birth{
-				BirthMonth: 10,
-				BirthDay:   18,
+				BirthMonth: ss.UserPref.EffectiveBirthMonth(),
+				BirthDay:   ss.UserPref.EffectiveBirthDay(),
 			},
 			ServerTimestamp: time.Now().Unix(),
 		},