feat(cli): Stammdaten-TUI mit Master Data API-Anbindung

- Neue Screens: Kategorien, Lieferanten, Artikel, Kunden (jeweils
  Liste, Detail, Anlegen + Detailaktionen wie Bewertung, Zertifikate,
  Verkaufseinheiten, Lieferadressen, Präferenzen)
- API-Client: Resources für alle 4 Stammdaten-Aggregate implementiert
  (categories, suppliers, articles, customers) mit Mapping von
  verschachtelten Domain-VOs auf flache DTOs
- Lieferant, Artikel, Kategorie: echte HTTP-Calls gegen Backend
  (/api/suppliers, /api/articles, /api/categories, /api/customers)
- 204-No-Content-Endpoints (removeSalesUnit, removeSupplier,
  removeCertificate, removeDeliveryAddress, removeFrameContract)
  lösen Re-Fetch des Aggregats aus
- MasterdataMenu, Navigation-Erweiterung, App.tsx-Routing

frontend/apps/cli/src/components/masterdata/suppliers/SupplierCreateScreen.tsx

@@ -0,0 +1,123 @@
+import React, { useState } from 'react';
+import { Box, Text, useInput } from 'ink';
+import { useNavigation } from '../../../state/navigation-context.js';
+import { useSuppliers } from '../../../hooks/useSuppliers.js';
+import { FormInput } from '../../shared/FormInput.js';
+import { LoadingSpinner } from '../../shared/LoadingSpinner.js';
+import { ErrorDisplay } from '../../shared/ErrorDisplay.js';
+
+type Field = 'name' | 'phone' | 'email' | 'contactPerson' | 'street' | 'houseNumber' | 'postalCode' | 'city' | 'country' | 'paymentDueDays';
+const FIELDS: Field[] = ['name', 'phone', 'email', 'contactPerson', 'street', 'houseNumber', 'postalCode', 'city', 'country', 'paymentDueDays'];
+
+const FIELD_LABELS: Record<Field, string> = {
+  name: 'Name *',
+  phone: 'Telefon *',
+  email: 'E-Mail',
+  contactPerson: 'Ansprechpartner',
+  street: 'Straße',
+  houseNumber: 'Hausnummer',
+  postalCode: 'PLZ',
+  city: 'Stadt',
+  country: 'Land',
+  paymentDueDays: 'Zahlungsziel (Tage)',
+};
+
+export function SupplierCreateScreen() {
+  const { navigate, back } = useNavigation();
+  const { createSupplier, loading, error, clearError } = useSuppliers();
+
+  const [values, setValues] = useState<Record<Field, string>>({
+    name: '', phone: '', email: '', contactPerson: '',
+    street: '', houseNumber: '', postalCode: '', city: '', country: 'Deutschland',
+    paymentDueDays: '',
+  });
+  const [activeField, setActiveField] = useState<Field>('name');
+  const [fieldErrors, setFieldErrors] = useState<Partial<Record<Field, string>>>({});
+
+  const setField = (field: Field) => (value: string) => {
+    setValues((v) => ({ ...v, [field]: value }));
+  };
+
+  useInput((_input, key) => {
+    if (loading) return;
+    if (key.tab || key.downArrow) {
+      setActiveField((f) => {
+        const idx = FIELDS.indexOf(f);
+        return FIELDS[(idx + 1) % FIELDS.length] ?? f;
+      });
+    }
+    if (key.upArrow) {
+      setActiveField((f) => {
+        const idx = FIELDS.indexOf(f);
+        return FIELDS[(idx - 1 + FIELDS.length) % FIELDS.length] ?? f;
+      });
+    }
+    if (key.escape) back();
+  });
+
+  const handleSubmit = async () => {
+    const errors: Partial<Record<Field, string>> = {};
+    if (!values.name.trim()) errors.name = 'Name ist erforderlich.';
+    if (!values.phone.trim()) errors.phone = 'Telefon ist erforderlich.';
+    setFieldErrors(errors);
+    if (Object.keys(errors).length > 0) return;
+
+    const result = await createSupplier({
+      name: values.name.trim(),
+      phone: values.phone.trim(),
+      ...(values.email.trim() ? { email: values.email.trim() } : {}),
+      ...(values.contactPerson.trim() ? { contactPerson: values.contactPerson.trim() } : {}),
+      ...(values.street.trim() ? { street: values.street.trim() } : {}),
+      ...(values.houseNumber.trim() ? { houseNumber: values.houseNumber.trim() } : {}),
+      ...(values.postalCode.trim() ? { postalCode: values.postalCode.trim() } : {}),
+      ...(values.city.trim() ? { city: values.city.trim() } : {}),
+      ...(values.country.trim() ? { country: values.country.trim() } : {}),
+      ...(values.paymentDueDays.trim() ? { paymentDueDays: parseInt(values.paymentDueDays, 10) } : {}),
+    });
+    if (result) navigate('supplier-list');
+  };
+
+  const handleFieldSubmit = (field: Field) => (_value: string) => {
+    const idx = FIELDS.indexOf(field);
+    if (idx < FIELDS.length - 1) {
+      setActiveField(FIELDS[idx + 1] ?? field);
+    } else {
+      void handleSubmit();
+    }
+  };
+
+  if (loading) {
+    return (
+      <Box flexDirection="column" alignItems="center" paddingY={2}>
+        <LoadingSpinner label="Lieferant wird angelegt..." />
+      </Box>
+    );
+  }
+
+  return (
+    <Box flexDirection="column" gap={1}>
+      <Text color="cyan" bold>Neuer Lieferant</Text>
+      {error && <ErrorDisplay message={error} onDismiss={clearError} />}
+
+      <Box flexDirection="column" gap={1} width={60}>
+        {FIELDS.map((field) => (
+          <FormInput
+            key={field}
+            label={FIELD_LABELS[field]}
+            value={values[field]}
+            onChange={setField(field)}
+            onSubmit={handleFieldSubmit(field)}
+            focus={activeField === field}
+            {...(fieldErrors[field] ? { error: fieldErrors[field] } : {})}
+          />
+        ))}
+      </Box>
+
+      <Box marginTop={1}>
+        <Text color="gray" dimColor>
+          Tab/↑↓ Feld wechseln · Enter auf letztem Feld speichern · Escape Abbrechen
+        </Text>
+      </Box>
+    </Box>
+  );
+}