Implement tabbed calendar management modal with improved styling

- Replace separate Create Calendar and External Calendar modals with unified tabbed interface
- Redesign modal styling with less rounded corners and cleaner appearance
- Significantly increase padding throughout modal components for better spacing
- Fix CSS variable self-references (control-padding, standard-transition)
- Improve button styling with better padding (0.875rem 2rem) and colors
- Enhance form elements with generous padding (1rem) and improved focus states
- Redesign tab bar with segmented control appearance and proper active states
- Update context menus with modern glassmorphism styling and smooth animations
- Consolidate calendar management functionality into single reusable component

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

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

frontend/src/components/sidebar.rs

@@ -100,8 +100,7 @@ impl Default for ViewMode {
 pub struct SidebarProps {
     pub user_info: Option<UserInfo>,
     pub on_logout: Callback<()>,
-    pub on_create_calendar: Callback<()>,
-    pub on_create_external_calendar: Callback<()>,
+    pub on_add_calendar: Callback<()>,
     pub external_calendars: Vec<ExternalCalendar>,
     pub on_external_calendar_toggle: Callback<i32>,
     pub on_external_calendar_delete: Callback<i32>,
@@ -357,12 +356,8 @@ pub fn sidebar(props: &SidebarProps) -> Html {
             </div>
             
             <div class="sidebar-footer">
-                <button onclick={props.on_create_calendar.reform(|_| ())} class="create-calendar-button">
-                    {"+ Create Calendar"}
-                </button>
-                
-                <button onclick={props.on_create_external_calendar.reform(|_| ())} class="create-external-calendar-button">
-                    {"+ Add External Calendar"}
+                <button onclick={props.on_add_calendar.reform(|_| ())} class="add-calendar-button">
+                    {"+ Add Calendar"}
                 </button>
 
                 <div class="view-selector">