redesigned payment page + a lot of fixes

frontend/pweb/lib/controllers/payments/amount_field.dart

@@ -0,0 +1,67 @@
+import 'package:flutter/material.dart';
+
+import 'package:pshared/provider/payment/amount.dart';
+import 'package:pshared/utils/currency.dart';
+import 'package:pshared/utils/money.dart';
+
+
+class PaymentAmountFieldController extends ChangeNotifier {
+  final TextEditingController textController;
+  PaymentAmountProvider? _provider;
+  bool _isSyncingText = false;
+
+  PaymentAmountFieldController({required double initialAmount})
+      : textController = TextEditingController(
+          text: amountToString(initialAmount),
+        );
+
+  void update(PaymentAmountProvider provider) {
+    if (identical(_provider, provider)) return;
+    _provider?.removeListener(_handleProviderChanged);
+    _provider = provider;
+    _provider?.addListener(_handleProviderChanged);
+    _syncTextWithAmount(provider.amount);
+  }
+
+  void handleChanged(String value) {
+    if (_isSyncingText) return;
+    final parsed = _parseAmount(value);
+    if (parsed != null) {
+      _provider?.setAmount(parsed);
+    }
+  }
+
+  void _handleProviderChanged() {
+    final provider = _provider;
+    if (provider == null) return;
+    _syncTextWithAmount(provider.amount);
+  }
+
+  double? _parseAmount(String value) {
+    final parsed = parseMoneyAmount(
+      value.replaceAll(',', '.'),
+      fallback: double.nan,
+    );
+    return parsed.isNaN ? null : parsed;
+  }
+
+  void _syncTextWithAmount(double amount) {
+    final parsedText = _parseAmount(textController.text);
+    if (parsedText != null && parsedText == amount) return;
+
+    final nextText = amountToString(amount);
+    _isSyncingText = true;
+    textController.value = TextEditingValue(
+      text: nextText,
+      selection: TextSelection.collapsed(offset: nextText.length),
+    );
+    _isSyncingText = false;
+  }
+
+  @override
+  void dispose() {
+    _provider?.removeListener(_handleProviderChanged);
+    textController.dispose();
+    super.dispose();
+  }
+}