fix(reminders): settings UI discloses that automatic sending is disabled (#2033)

* fix(reminders): settings UI discloses that automatic sending is disabled

The invoice reminder cron has answered 503 since May 2026 (PR #583), so
no automatic reminders are sent, but the settings UI still let users
configure reminder day levels as if sending worked.

Introduce REMINDERS_SENDING_ENABLED (lib/invoices/reminders-enabled.ts)
as the single shared flag read by both sides: the cron route uses it as
its 503 gate (with the original sending pipeline restored behind it, so
re-enabling later is one flag flip), and the invoice settings form shows
an attn notice while the flag is off. Schedule settings stay editable;
notice strings added to both sv and en locales.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NtvffGr6uVk2J2Skuz6L98

* docs(reminders): correct the re-enable contract after skeptic review

The flag docblock, route docblock, and test header claimed flipping
REMINDERS_SENDING_ENABLED alone resumes sending. False on hosted: the
route has had no vercel.json cron entry since PR #559 and the crontab
ratchet pins it in INTENTIONALLY_UNSCHEDULED, and POST requires the cron
secret so no dashboard can trigger it. Rewrite the claims into the real
re-enable checklist and record the pre-flip prerequisites surfaced by
review: invoice_reminders lacks a unique (invoice_id, reminder_level)
constraint and the fee entry is booked before the reminder row, so a run
dying mid-batch double-books the fee; the backlog would get highest-level
reminders first. Comments and a test name only; no runtime change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NtvffGr6uVk2J2Skuz6L98

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/settings/InvoiceSettingsForm.tsx

@@ -3,6 +3,7 @@
 import { useState } from 'react'
 import { useTranslations } from 'next-intl'
 import { Switch } from '@/components/ui/switch'
+import { AttnLine } from '@/components/ui/attn-line'
 import {
   SettingsGroup,
   SettingsInput,
@@ -10,6 +11,7 @@ import {
   SettingsRow,
   SettingsTextarea,
 } from '@/components/settings/SettingsRows'
+import { REMINDERS_SENDING_ENABLED } from '@/lib/invoices/reminders-enabled'
 import type { CompanySettings } from '@/types'
 
 interface InvoiceSettingsFormProps {
@@ -104,6 +106,9 @@ export function InvoiceSettingsForm({ settings }: InvoiceSettingsFormProps) {
       </SettingsGroup>
 
       <SettingsGroup label={t('reminder_days_heading')} help={t('reminder_days_help')}>
+        {!REMINDERS_SENDING_ENABLED && (
+          <AttnLine className="px-1 pt-2">{t('sending_disabled_notice')}</AttnLine>
+        )}
         <SettingsRow
           label={t('send_reminders_label')}
           htmlFor="send_invoice_reminders"