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/db.js

@@ -1180,6 +1180,15 @@ const MIGRATIONS = [
       CREATE INDEX idx_contact_addresses_contact ON contact_addresses(contact_id);
     `,
   },
+  {
+    version: 31,
+    description: 'Advanced reminder options for birthdays',
+    up: `
+      ALTER TABLE birthdays ADD COLUMN reminder_offset TEXT;
+      ALTER TABLE birthdays ADD COLUMN reminder_custom_amount INTEGER;
+      ALTER TABLE birthdays ADD COLUMN reminder_custom_unit TEXT;
+    `,
+  },
 ];
 
 /**