Add comprehensive iCal properties support to event creation modal

Enhanced the create event modal to include all major iCalendar properties:
- Event status (confirmed/tentative/cancelled)
- Privacy classification (public/private/confidential)
- Priority levels (0-9 numeric scale)
- Organizer email field
- Attendees list (comma-separated emails)
- Categories (comma-separated tags)
- Reminder options (none to 1 week before)
- Recurrence patterns (none/daily/weekly/monthly/yearly)

Updated backend to parse and handle all new fields, with proper enum conversion
and comma-separated list parsing. Events now generate complete iCal data with
STATUS, CLASS, PRIORITY, ORGANIZER, ATTENDEE, CATEGORIES, VALARM, and RRULE properties.

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

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

backend/src/models.rs

@@ -80,6 +80,14 @@ pub struct CreateEventRequest {
     pub end_time: String,          // HH:MM format
     pub location: String,
     pub all_day: bool,
+    pub status: String,            // confirmed, tentative, cancelled
+    pub class: String,             // public, private, confidential
+    pub priority: Option<u8>,      // 0-9 priority level
+    pub organizer: String,         // organizer email
+    pub attendees: String,         // comma-separated attendee emails
+    pub categories: String,        // comma-separated categories
+    pub reminder: String,          // reminder type
+    pub recurrence: String,        // recurrence type
     pub calendar_path: Option<String>, // Optional - use first calendar if not specified
 }