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/services/calendar_service.rs

@@ -607,6 +607,7 @@ impl CalendarService {
         categories: String,
         reminder: String,
         recurrence: String,
+        recurrence_days: Vec<bool>,
         calendar_path: Option<String>
     ) -> Result<(), String> {
         let window = web_sys::window().ok_or("No global window exists")?;
@@ -632,6 +633,7 @@ impl CalendarService {
             "categories": categories,
             "reminder": reminder,
             "recurrence": recurrence,
+            "recurrence_days": recurrence_days,
             "calendar_path": calendar_path
         });