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/payment/productlist.go

@@ -21,7 +21,7 @@ func productList(ctx *gin.Context) {
 				Restricted: false,
 			},
 			UnderAgeInfo: paymentapischema.UnderAgeInfo{
-				BirthSet:    true,
+				BirthSet:    ss.UserPref.HasBirthDate(),
 				HasLimit:    false,
 				LimitAmount: nil,
 				MonthUsed:   0,