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>

src/app.rs

@@ -272,6 +272,7 @@ pub fn App() -> Html {
                         event_data.categories,
                         reminder_str,
                         recurrence_str,
+                        event_data.recurrence_days,
                         None // Let backend use first available calendar
                     ).await {
                         Ok(_) => {