Implement comprehensive external calendar event deduplication and fixes

- Add UID-based deduplication to prefer recurring events over single events with same UID
- Implement RRULE-generated instance detection to filter duplicate occurrences
- Add title normalization for case-insensitive matching and consolidation
- Fix external calendar refresh button with proper error handling and loading states
- Update context menu for external events to show only "View Event Details" option
- Add comprehensive multi-pass deduplication: UID → title consolidation → RRULE filtering

This resolves issues where Outlook calendars showed duplicate events with same UID
but different RRULE states (e.g., "Dragster Stand Up" appearing both as recurring
and single events).

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

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

frontend/src/components/sidebar.rs

@@ -110,6 +110,7 @@ pub struct SidebarProps {
     pub on_color_change: Callback<(String, String)>,
     pub on_color_picker_toggle: Callback<String>,
     pub available_colors: Vec<String>,
+    pub refreshing_calendar_id: Option<i32>,
     pub on_calendar_context_menu: Callback<(MouseEvent, String)>,
     pub on_calendar_visibility_toggle: Callback<String>,
     pub current_view: ViewMode,
@@ -304,8 +305,15 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                                                                     on_refresh.emit(cal_id);
                                                                 })
                                                             }}
+                                                            disabled={props.refreshing_calendar_id == Some(cal.id)}
                                                         >
-                                                            {"🔄"}
+                                                            {
+                                                                if props.refreshing_calendar_id == Some(cal.id) {
+                                                                    "⏳" // Loading spinner
+                                                                } else {
+                                                                    "🔄" // Normal refresh icon
+                                                                }
+                                                            }
                                                         </button>
                                                     </div>
                                                 </div>