Improve sats numeric entry for zaps

Changelog-Changed: Add number formatting for sats entry and use selected zaps amount from picker as placeholder
Changelog-Fixed: Do not allow non-numeric characters for sats amount and fix numeric entry for other number systems for all locales

damus/Models/UserSettingsStore.swift

@@ -26,11 +26,13 @@ func set_default_zap_amount(pubkey: String, amount: Int) {
     UserDefaults.standard.setValue(amount, forKey: key)
 }
 
-func get_default_zap_amount(pubkey: String) -> Int? {
+let fallback_zap_amount = 1000
+
+func get_default_zap_amount(pubkey: String) -> Int {
     let key = default_zap_setting_key(pubkey: pubkey)
     let amt = UserDefaults.standard.integer(forKey: key)
     if amt == 0 {
-        return nil
+        return fallback_zap_amount
     }
     return amt
 }