Add Kundennummer/Nummernkreise settings and settings dialog.

Add number & setting API endpoints, StorageManager caches, and a Settings
dialog (tabs for settings keys and number ranges) reachable from the user
menu. Show Kundennummer and creation date in the Kunde list, and refactor
Neuer Kunde to show an auto-generated number with group select.

src/api/index.js

@@ -772,6 +772,11 @@ export const entity = {
     return http(`/api/entity/${id}`, { method: 'DELETE' });
   },
 
+  /** GET /api/entity/next-number → { number: string } */
+  nextNumber() {
+    return http('/api/entity/next-number');
+  },
+
   /** GET /api/entity/:entityId/address → { addresses: EntityAddress[] } */
   listAddresses(entityId) {
     return http(`/api/entity/${entityId}/address`);
@@ -936,6 +941,25 @@ export const posterminal = {
   },
 };
 
+export const number = {
+  /**
+   * GET /api/number → { numbers: [{ id, type, prefix, padding, next }] }
+   * Lists the number sequences for the selected company.
+   */
+  list() {
+    return http('/api/number');
+  },
+
+  /**
+   * PATCH /api/number/:id
+   * Body: at least one of { prefix, padding, next } — only present keys are patched.
+   * → 200 { number: { id, type, prefix, padding, next } }
+   */
+  update(id, body) {
+    return http(`/api/number/${id}`, { method: 'PATCH', body });
+  },
+};
+
 export const auth = {
   async me() {
     try {
@@ -997,6 +1021,7 @@ export default {
   customergroup,
   delivery,
   entity,
+  number,
   order,
   posterminal,
   quote,