feat: add flexible reminder options for birthdays

Add support for customizable birthday reminders with preset offsets
(none, at time, 15min, 1h, 1d, 2d, 1w, 2w) and custom intervals.
Users can now configure when to be reminded of upcoming birthdays.

- Add migration 31: reminder_offset, reminder_custom_amount, reminder_custom_unit to birthdays table
- Update POST/PUT /birthdays routes to accept reminder fields
- Add getOffsetMinutes() helper in birthday service
- Update birthdayReminderAt() to calculate reminder time with offset
- Modify syncBirthdayReminder() to handle empty offset (no reminder)
- Add renderBirthdayReminderSection() UI component
- Move reminder-custom CSS from calendar.css to reminders.css
- Add protocol check to service worker (non-http protocol guard)

All translations already present in de.json.
Tests: 109 passing, 0 failing.

Co-Authored-By: Rafael Foster <rafaelfoster@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

server/services/birthdays.js

@@ -43,9 +43,22 @@ function daysUntilBirthday(birthDate, from = new Date()) {
   return Math.round((nextUtc - todayUtc) / 86400000);
 }
 
-function birthdayReminderAt(birthDate, from = new Date()) {
+function getOffsetMinutes(birthday) {
+  if (birthday.reminder_offset === 'custom') {
+    const amount = parseInt(birthday.reminder_custom_amount, 10) || 1;
+    const unit = birthday.reminder_custom_unit || 'days';
+    if (unit === 'weeks') return amount * 10080;
+    if (unit === 'days') return amount * 1440;
+    if (unit === 'hours') return amount * 60;
+    return amount;
+  }
+  return parseInt(birthday.reminder_offset, 10) || 0;
+}
+
+function birthdayReminderAt(birthDate, offsetMin = 0, from = new Date()) {
   const next = nextBirthdayDate(birthDate, from);
-  return `${next}T12:00:00Z`;
+  const baseTime = new Date(`${next}T12:00:00Z`).getTime();
+  return new Date(baseTime - (offsetMin || 0) * 60000).toISOString();
 }
 
 function eventTitle(name) {
@@ -125,7 +138,16 @@ function syncBirthdayCalendarEvent(database, birthday) {
 function syncBirthdayReminder(database, birthday, from = new Date()) {
   if (!birthday.calendar_event_id) return null;
 
-  const desired = birthdayReminderAt(birthday.birth_date, from);
+  if (birthday.reminder_offset === '') {
+    database.prepare(`
+      DELETE FROM reminders
+      WHERE entity_type = 'event' AND entity_id = ? AND created_by = ?
+    `).run(birthday.calendar_event_id, birthday.created_by);
+    return null;
+  }
+
+  const offsetMin = getOffsetMinutes(birthday);
+  const desired = birthdayReminderAt(birthday.birth_date, offsetMin, from);
   const existing = database.prepare(`
     SELECT * FROM reminders
     WHERE entity_type = 'event' AND entity_id = ? AND created_by = ?