refactor(accounting): consolidate the Accounting tab into two cards + one Save

- Box 1 "Default rates": mileage, daily allowance, hourly rate, require-proof.
  Hints now make the cost-vs-billing split explicit (daily allowance = expense,
  hourly = billing fallback).
- Box 2 retitled "VAT": registration, reclaim, default invoice VAT code, and
  the VAT label (moved out of its own card).
- Drop the third card (AccountingProfileFields deleted); the two Save buttons
  become one — it persists both the app_settings and the two business_profile
  fields (VAT label + hourly rate) together.
- Rename "Per-diem" → "Daily allowance" (EN) for clarity; German keeps the
  established "Spesenpauschale".

frontend/src/components/admin/AccountingProfileFields.tsx

@@ -1,82 +0,0 @@
-/**
- * Business-profile financial fields surfaced on the Accounting tab: the
- * **VAT label** (printed on invoice/quote PDFs) and the **default hourly rate**
- * (install-wide fallback for hours logging). The values still live on
- * `business_profile`; this is a self-contained card with its own save (mirrors
- * VatCodesManager) so it can't clobber the rest of the business profile, and it
- * shares the `business-profile` query cache so both pages stay in sync.
- */
-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, Input, Loading } from '../common';
-import { DecimalInput } from '../common/DecimalInput';
-import { businessProfileService } from '../../services/businessProfile.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 AccountingProfileFields: React.FC = () => {
-  const { t } = useTranslation();
-  const qc = useQueryClient();
-  const { data, isLoading } = useQuery({ queryKey: ['business-profile'], queryFn: () => businessProfileService.get() });
-
-  const [vatLabel, setVatLabel] = useState('');
-  const [hourlyMajor, setHourlyMajor] = useState<number>(NaN);
-  const currency = data?.profile?.defaultCurrency || 'CHF';
-
-  useEffect(() => {
-    if (data?.profile) {
-      setVatLabel(data.profile.vatLabel || '');
-      setHourlyMajor(data.profile.defaultHourlyRateMinor != null ? data.profile.defaultHourlyRateMinor / 100 : NaN);
-    }
-  }, [data]);
-
-  const save = useMutation({
-    mutationFn: () => businessProfileService.update({
-      vatLabel: vatLabel || '',
-      defaultHourlyRateMinor: Number.isFinite(hourlyMajor) ? Math.max(0, Math.round(hourlyMajor * 100)) : null,
-    }),
-    onSuccess: () => {
-      toast.success(t('settings.accounting.profileFields.savedToast', 'Saved.'));
-      qc.invalidateQueries({ queryKey: ['business-profile'] });
-    },
-    onError: (e: any) => toast.error(e?.response?.data?.error || e.message || 'Failed'),
-  });
-
-  if (isLoading) return <Loading />;
-
-  return (
-    <Card><CardContent className="p-5 space-y-4">
-      <h3 className="text-sm font-semibold uppercase tracking-wider text-neutral-500 dark:text-neutral-400">
-        {t('settings.accounting.profileFields.title', 'VAT label & hourly rate')}
-      </h3>
-
-      <div>
-        <label className={labelCls}>{t('settings.accounting.profileFields.vatLabel', 'VAT label (e.g. MwSt., VAT)')}</label>
-        <Input value={vatLabel} onChange={(e) => setVatLabel(e.target.value)} className={inputCls} />
-        <p className="mt-1 text-xs text-neutral-500 dark:text-neutral-400">{t('settings.accounting.profileFields.vatLabelHint', 'Printed as the VAT-line label on invoice / quote PDFs. Leave blank to use the document language default.')}</p>
-      </div>
-
-      <div>
-        <label className={labelCls}>{t('settings.accounting.profileFields.hourlyRate', 'Default hourly rate')}</label>
-        <DecimalInput
-          value={hourlyMajor}
-          fractionDigits={2}
-          onChange={setHourlyMajor}
-          className={inputCls}
-          placeholder={t('settings.accounting.profileFields.hourlyRatePlaceholder', 'e.g. 120.00') as string}
-        />
-        <p className="mt-1 text-xs text-neutral-500 dark:text-neutral-400">
-          {t('settings.accounting.profileFields.hourlyRateHint', 'Fallback used when a customer has no own rate. In {{currency}}, major units. Leave blank to require a per-customer or per-entry rate.', { currency })}
-        </p>
-      </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>
-    </CardContent></Card>
-  );
-};
-
-export default AccountingProfileFields;