fix(crm): localize scheduled-send + installment date + timezone picker

From dev testing:
- BillEditor 'Geplanter Versand' was a native <input type=datetime-local> →
  rendered US date + 12h regardless of settings. Split into LocalizedDateInput
  + TimeField (honour general_date_format + general_time_format), recombined
  into the YYYY-MM-DDTHH:MM the payload/scheduler expect.
- InstallmentsPanel 'Send on' native <input type=date> (browser-locale via a
  lang hint, wrong in Safari/Firefox) → LocalizedDateInput, consistent in every
  browser. (Luca approved converting it.)
- Business-profile Timezone was a free-text input → dropdown of the full IANA
  list (Intl.supportedValuesOf, CH/LI fallback), blank = system default.

frontend/src/components/admin/InstallmentsPanel.tsx

@@ -26,10 +26,9 @@
 import React, { useMemo } from 'react';
 import { useTranslation } from 'react-i18next';
 import { Trash2, Plus } from 'lucide-react';
-import { Button, Input } from '../common';
+import { Button, Input, LocalizedDateInput } from '../common';
 import type { PaymentTermInstallment } from '../../services/quotes.service';
 import { useInstallmentDefaults } from '../../hooks/useInstallmentDefaults';
-import { useLocalizedDate } from '../../hooks/useLocalizedDate';
 
 export type InstallmentPlan = PaymentTermInstallment[];
 
@@ -69,7 +68,6 @@ export const InstallmentsPanel: React.FC<InstallmentsPanelProps> = ({
   value, onChange, onValidityChange, eventDate, disabled,
 }) => {
   const { t } = useTranslation();
-  const { dateInputLang } = useLocalizedDate();
   const defaults = useInstallmentDefaults();
   const [advanced, setAdvanced] = React.useState(false);
 
@@ -230,12 +228,9 @@ export const InstallmentsPanel: React.FC<InstallmentsPanelProps> = ({
                           'On delivery — admin releases manually. Switch to advanced to change.')}
                       </div>
                     ) : (
-                      <Input
-                        type="date"
-                        lang={dateInputLang}
+                      <LocalizedDateInput
                         value={previewDate(row) || ''}
-                        onChange={(e) => {
-                          const next = e.target.value;
+                        onChange={(next) => {
                           if (!next) return;
                           const offset = daysBetween(todayIso(), next);
                           update(idx, { trigger: 'fixed_date', offset_days: offset });

frontend/src/pages/admin/bills/BillEditorPage.tsx

@@ -566,8 +566,31 @@ export const BillEditorPage: React.FC = () => {
                 : t('bills.field.dueDateOverrideOff', 'Auto from send date + payment term — tick to set manually')}
             </label>
           </div>
-          <Input type="datetime-local" label={t('bills.field.scheduledSendAt', 'Scheduled send (optional)') as string}
-            value={scheduledSendAt} onChange={(e) => setScheduledSendAt(e.target.value)} />
+          <div>
+            <label className="block text-sm font-medium mb-1">{t('bills.field.scheduledSendAt', 'Scheduled send (optional)')}</label>
+            {/* Localized date + time (honours general_date_format +
+                general_time_format) instead of a native datetime-local, which
+                renders in the browser locale (US date + 12h). Recombined into
+                the "YYYY-MM-DDTHH:MM" the payload + scheduler expect. */}
+            <div className="grid grid-cols-2 gap-3">
+              <LocalizedDateInput
+                value={scheduledSendAt ? scheduledSendAt.slice(0, 10) : ''}
+                onChange={(iso) => {
+                  if (!iso) { setScheduledSendAt(''); return; }
+                  const time = scheduledSendAt.length >= 16 ? scheduledSendAt.slice(11, 16) : '09:00';
+                  setScheduledSendAt(`${iso}T${time}`);
+                }}
+              />
+              <TimeField
+                value={scheduledSendAt.length >= 16 ? scheduledSendAt.slice(11, 16) : ''}
+                onChange={(hhmm) => {
+                  const date = scheduledSendAt ? scheduledSendAt.slice(0, 10) : '';
+                  if (!date) return;
+                  setScheduledSendAt(`${date}T${hhmm || '09:00'}`);
+                }}
+              />
+            </div>
+          </div>
           <div>
             <label className="block text-sm font-medium mb-1">{t('bills.field.qrFormat', 'Payment QR format')}</label>
             <select

frontend/src/pages/admin/settings/SettingsBusinessProfilePage.tsx

@@ -21,6 +21,18 @@ import { Button, Card, Loading, Input, CountrySelect, TimeField } from '../../..
 import { DecimalInput } from '../../../components/common/DecimalInput';
 import { toast } from 'react-toastify';
 
+// Full IANA timezone list for the picker. `Intl.supportedValuesOf` is ES2022
+// (all current browsers); fall back to a small CH/LI-relevant set on the rare
+// engine that lacks it.
+const IANA_TIMEZONES: string[] = (() => {
+  try {
+    // @ts-expect-error supportedValuesOf is ES2022, not yet in all TS lib defs
+    return Intl.supportedValuesOf('timeZone') as string[];
+  } catch {
+    return ['UTC', 'Europe/Vaduz', 'Europe/Zurich', 'Europe/Berlin', 'Europe/Vienna', 'Europe/Paris', 'Europe/London'];
+  }
+})();
+
 export const SettingsBusinessProfilePage: React.FC = () => {
   const { t } = useTranslation();
   const qc = useQueryClient();
@@ -117,17 +129,26 @@ export const SettingsBusinessProfilePage: React.FC = () => {
             maxLength={3} onChange={(e) => setProfile({ ...profile, defaultCurrency: e.target.value.toUpperCase() })} />
           <Input label={t('businessProfile.field.defaultLocale', 'Default locale') as string} value={profile.defaultLocale}
             maxLength={8} onChange={(e) => setProfile({ ...profile, defaultLocale: e.target.value })} />
-          {/* Migration 137 — IANA timezone string for the admin calendar.
-              Free-text; backend caps at 64 chars. When blank the calendar
-              UI falls back to the browser's `Intl.DateTimeFormat()
-              .resolvedOptions().timeZone`. */}
-          <Input
-            label={t('businessProfile.field.timezone', 'Timezone (IANA)') as string}
+          {/* Migration 137 — IANA timezone for the admin calendar + the
+              scheduled-email business-hours snapping. Dropdown of the full
+              IANA list; blank = fall back to the server/browser tz. */}
+          <div>
+            <label className="block text-sm font-medium text-neutral-700 dark:text-neutral-300 mb-1">
+              {t('businessProfile.field.timezone', 'Timezone (IANA)')}
+            </label>
+            <select
               value={profile.timezone || ''}
-            maxLength={64}
-            placeholder={Intl.DateTimeFormat().resolvedOptions().timeZone}
               onChange={(e) => setProfile({ ...profile, timezone: e.target.value || null })}
-          />
+              className="w-full px-3 py-2 rounded-md border border-neutral-300 dark:border-neutral-600 bg-white dark:bg-neutral-800 text-sm focus:outline-none focus:ring-2 focus:ring-primary-500"
+            >
+              <option value="">
+                {t('businessProfile.field.timezoneSystemDefault', 'System default')} ({Intl.DateTimeFormat().resolvedOptions().timeZone})
+              </option>
+              {IANA_TIMEZONES.map((tz) => (
+                <option key={tz} value={tz}>{tz}</option>
+              ))}
+            </select>
+          </div>
           <Input label={t('businessProfile.field.vatLabel', 'VAT label (e.g. MwSt., VAT)') as string} value={profile.vatLabel}
             onChange={(e) => setProfile({ ...profile, vatLabel: e.target.value })} />
           <Input type="number" step="0.01" label={t('businessProfile.field.vatRateDefault', 'Default VAT rate %') as string}