Implement comprehensive theme system with calendar view support

- Add 8 attractive themes (Default, Ocean, Forest, Sunset, Purple, Dark, Rose, Mint)
- Extend theme system to calendar header, month view, and week view components
- Add dynamic theme-aware event color palettes that change with selected theme
- Implement CSS custom properties for consistent theming across all components
- Add localStorage persistence for user theme preferences
- Create theme-aware calendar styling including day states, headers, and grid lines
- Optimize dark theme with proper contrast and readability improvements
- Add reactive event color system that updates when themes change

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

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

backend/src/models.rs

@@ -124,6 +124,9 @@ pub struct UpdateEventRequest {
     pub calendar_path: Option<String>, // Optional - search all calendars if not specified
     pub update_action: Option<String>, // "update_series" for recurring events
     pub occurrence_date: Option<String>, // ISO date string for specific occurrence
+    pub exception_dates: Option<Vec<String>>, // ISO datetime strings for EXDATE
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub until_date: Option<String>, // ISO datetime string for RRULE UNTIL clause
 }
 
 #[derive(Debug, Serialize)]