Replace page reloads with dynamic calendar refresh functionality

- Add refresh_calendar_data function to replace window.location.reload()
- Implement dynamic event re-fetching without full page refresh
- Add last_updated timestamp to UserInfo to force component re-renders
- Fix WASM compatibility by using js_sys::Date::now() instead of SystemTime
- Remove debug logging from refresh operations
- Maintain same user experience with improved performance

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

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

frontend/src/services/calendar_service.rs

@@ -37,6 +37,12 @@ pub struct UserInfo {
     pub username: String,
     pub server_url: String,
     pub calendars: Vec<CalendarInfo>,
+    #[serde(default = "default_timestamp")]
+    pub last_updated: u64,
+}
+
+fn default_timestamp() -> u64 {
+    0
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]