Add weekday selection for weekly recurrence and fix RRULE generation

- Add weekday selection UI for weekly recurring events with checkboxes
- Implement BYDAY parameter generation in RRULE based on selected days
- Fix missing RRULE generation in iCalendar output
- Convert reminder durations to proper EventReminder structs
- Add responsive CSS styling for weekday selection interface

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/calendar.rs

@@ -866,6 +866,11 @@ impl CalDAVClient {
             ical.push_str("END:VALARM\r\n");
         }
         
+        // Recurrence rule
+        if let Some(rrule) = &event.recurrence_rule {
+            ical.push_str(&format!("RRULE:{}\r\n", rrule));
+        }
+        
         ical.push_str("END:VEVENT\r\n");
         ical.push_str("END:VCALENDAR\r\n");