feat(accounting): Accounting settings tab (km / per-diem rate, require-proof)

New Settings -> Accounting tab (gated by the accounting flag) to edit the km
rate, per-diem rate and the "require proof for expense" toggle (reads GET /
writes PUT /admin/settings/accounting). Rates are CHF, stored as integer minor
units; carries the "verify with your Treuhaender" disclaimer. Wired into
SettingsPage (TabType, keys, flag-gated nav item, render) + the features barrel.

i18n: settings.accounting.* (EN + DE, DE native).
Verified: tsc --noEmit clean (0 errors); en/de JSON valid; npm run build green.

frontend/src/features/settings/index.ts

@@ -18,3 +18,4 @@ export { SEOTab } from './tabs/SEOTab';
 export { ThumbnailsTab } from './tabs/ThumbnailsTab';
 export { ApiTokensTab } from './tabs/ApiTokensTab';
 export { WebhooksTab } from './tabs/WebhooksTab';
+export { AccountingTab } from './tabs/AccountingTab';

frontend/src/features/settings/tabs/AccountingTab.tsx

@@ -0,0 +1,79 @@
+/**
+ * Accounting settings tab — rates used by internal expenses + the proof
+ * requirement. Rates are CHF; stored as integer minor units. Tax/legal
+ * guidance only — verify with your Treuhaender.
+ */
+import React, { useEffect, useState } from 'react';
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { useTranslation } from 'react-i18next';
+import { toast } from 'react-toastify';
+import { Save } from 'lucide-react';
+import { Button, Card, CardContent, Loading } from '../../../components/common';
+import { DecimalInput } from '../../../components/common/DecimalInput';
+import { accountingService } from '../../../services/accounting.service';
+
+const labelCls = 'block text-sm font-medium text-neutral-700 dark:text-neutral-300 mb-1';
+const inputCls = 'w-full max-w-xs rounded-md border border-neutral-300 dark:border-neutral-600 bg-white dark:bg-neutral-800 px-3 py-2 text-sm';
+
+export const AccountingTab: React.FC = () => {
+  const { t } = useTranslation();
+  const qc = useQueryClient();
+  const { data, isLoading } = useQuery({ queryKey: ['accounting-settings'], queryFn: () => accountingService.getSettings() });
+
+  const [kmMajor, setKmMajor] = useState<number>(NaN);
+  const [perDiemMajor, setPerDiemMajor] = useState<number>(NaN);
+  const [requireProof, setRequireProof] = useState(false);
+
+  useEffect(() => {
+    if (data) {
+      setKmMajor(data.accounting_km_rate_minor / 100);
+      setPerDiemMajor(data.accounting_per_diem_rate_minor / 100);
+      setRequireProof(data.accounting_require_proof);
+    }
+  }, [data]);
+
+  const save = useMutation({
+    mutationFn: () => accountingService.updateSettings({
+      accounting_km_rate_minor: Number.isFinite(kmMajor) ? Math.round(kmMajor * 100) : 0,
+      accounting_per_diem_rate_minor: Number.isFinite(perDiemMajor) ? Math.round(perDiemMajor * 100) : 0,
+      accounting_require_proof: requireProof,
+    }),
+    onSuccess: () => { toast.success(t('settings.accounting.savedToast', 'Accounting settings saved.')); qc.invalidateQueries({ queryKey: ['accounting-settings'] }); },
+    onError: (e: any) => toast.error(e?.response?.data?.error || e.message || 'Failed'),
+  });
+
+  if (isLoading) return <Loading />;
+
+  return (
+    <div className="space-y-6">
+      <div>
+        <h2 className="text-xl font-bold text-neutral-900 dark:text-neutral-100">{t('settings.accounting.title', 'Accounting')}</h2>
+        <p className="text-neutral-600 dark:text-neutral-400 mt-1">{t('settings.accounting.subtitle', 'Default rates for internal expenses and the proof requirement.')}</p>
+      </div>
+
+      <Card><CardContent className="p-5 space-y-4">
+        <div>
+          <label className={labelCls}>{t('settings.accounting.kmRate', 'Mileage rate (CHF / km)')}</label>
+          <DecimalInput value={kmMajor} onChange={setKmMajor} fractionDigits={2} className={inputCls} />
+          <p className="mt-1 text-xs text-neutral-500 dark:text-neutral-400">{t('settings.accounting.kmRateHint', 'Default applied to mileage expenses; overridable per entry.')}</p>
+        </div>
+        <div>
+          <label className={labelCls}>{t('settings.accounting.perDiemRate', 'Per-diem rate (CHF / day)')}</label>
+          <DecimalInput value={perDiemMajor} onChange={setPerDiemMajor} fractionDigits={2} className={inputCls} />
+          <p className="mt-1 text-xs text-neutral-500 dark:text-neutral-400">{t('settings.accounting.perDiemRateHint', 'Default applied to per-diem expenses; overridable per entry.')}</p>
+        </div>
+        <label className="flex items-center gap-2 text-sm text-neutral-800 dark:text-neutral-200">
+          <input type="checkbox" checked={requireProof} onChange={(e) => setRequireProof(e.target.checked)} className="rounded border-neutral-300" />
+          {t('settings.accounting.requireProof', 'Require a proof file on every expense')}
+        </label>
+        <p className="text-xs text-amber-600 dark:text-amber-400">{t('settings.accounting.disclaimer', 'Rates and VAT/tax treatment are guidance only — verify with your Treuhaender.')}</p>
+      </CardContent></Card>
+
+      <div>
+        <Button onClick={() => save.mutate()} disabled={save.isPending}><Save className="w-4 h-4 mr-2" /> {save.isPending ? t('common.saving', 'Saving…') : t('common.save', 'Save')}</Button>
+      </div>
+    </div>
+  );
+};
+
+export default AccountingTab;

frontend/src/i18n/locales/de.json

@@ -1705,6 +1705,17 @@
     },
     "crm": {
       "title": "CRM-Verhalten"
+    },
+    "accounting": {
+      "title": "Buchhaltung",
+      "subtitle": "Standardsätze für interne Aufwände und die Belegpflicht.",
+      "kmRate": "Kilometersatz (CHF / km)",
+      "kmRateHint": "Standard für Kilometer-Aufwände; pro Eintrag überschreibbar.",
+      "perDiemRate": "Spesenpauschale (CHF / Tag)",
+      "perDiemRateHint": "Standard für Pauschal-Aufwände; pro Eintrag überschreibbar.",
+      "requireProof": "Beleg für jeden Aufwand verlangen",
+      "disclaimer": "Sätze und MwSt-/Steuerbehandlung dienen nur als Orientierung — mit Ihrem Treuhänder prüfen.",
+      "savedToast": "Buchhaltungseinstellungen gespeichert."
     }
   },
   "branding": {

frontend/src/i18n/locales/en.json

@@ -1263,6 +1263,17 @@
     },
     "crm": {
       "title": "CRM behaviour"
+    },
+    "accounting": {
+      "title": "Accounting",
+      "subtitle": "Default rates for internal expenses and the proof requirement.",
+      "kmRate": "Mileage rate (CHF / km)",
+      "kmRateHint": "Default applied to mileage expenses; overridable per entry.",
+      "perDiemRate": "Per-diem rate (CHF / day)",
+      "perDiemRateHint": "Default applied to per-diem expenses; overridable per entry.",
+      "requireProof": "Require a proof file on every expense",
+      "disclaimer": "Rates and VAT/tax treatment are guidance only — verify with your Treuhänder.",
+      "savedToast": "Accounting settings saved."
     }
   },
   "analytics": {

frontend/src/pages/admin/SettingsPage.tsx

@@ -40,6 +40,7 @@ import {
   ThumbnailsTab,
   ApiTokensTab,
   WebhooksTab,
+  AccountingTab,
 } from '../../features/settings';
 import { EmailConfigPage } from './EmailConfigPage';
 import { BrandingPage } from './BrandingPage';
@@ -52,7 +53,7 @@ import { CrmSettingsPage } from './settings/CrmSettingsPage';
 import { ReminderTemplatesPage } from './settings/ReminderTemplatesPage';
 import { BlockLibraryPage } from './contracts/BlockLibraryPage';
 import { useFeatureFlags } from '../../contexts/FeatureFlagsContext';
-import { Briefcase, Receipt, ScrollText, Mail } from 'lucide-react';
+import { Briefcase, Receipt, ScrollText, Mail, Landmark } from 'lucide-react';
 
 // Tab keys driving the inner-nav. Must include every key used in
 // `navGroups` below and in the switch at the bottom of the component.
@@ -81,7 +82,8 @@ type TabType =
   | 'businessProfile'
   | 'crm'
   | 'contracts'
-  | 'reminderTemplates';
+  | 'reminderTemplates'
+  | 'accounting';
 
 interface NavItem {
   key: TabType;
@@ -101,7 +103,7 @@ const ALL_TAB_KEYS: TabType[] = [
   'security', 'imageSecurity', 'seo',
   'apiTokens', 'webhooks',
   'status', 'analytics', 'backup',
-  'businessProfile', 'crm', 'contracts', 'reminderTemplates',
+  'businessProfile', 'crm', 'contracts', 'reminderTemplates', 'accounting',
 ];
 
 function isValidTab(value: string | null): value is TabType {
@@ -251,6 +253,9 @@ export const SettingsPage: React.FC = () => {
         ...(flags.reminderEmails
           ? [{ key: 'reminderTemplates' as const,  label: t('settings.reminderTemplates.title', 'Reminder emails'), icon: Mail }]
           : []),
+        ...(flags.accounting
+          ? [{ key: 'accounting' as const,         label: t('settings.accounting.title',      'Accounting'),       icon: Landmark }]
+          : []),
       ],
     },
     {
@@ -414,6 +419,7 @@ export const SettingsPage: React.FC = () => {
           {activeTab === 'crm' && <CrmSettingsPage />}
           {activeTab === 'contracts' && <BlockLibraryPage />}
           {activeTab === 'reminderTemplates' && <ReminderTemplatesPage />}
+          {activeTab === 'accounting' && <AccountingTab />}
 
           {activeTab === 'status' && (
             <StatusTab