Implement smart context menu positioning

Added intelligent viewport boundary detection that repositions context
menus when they would appear outside the screen:

- Detects right/bottom edge overflow and repositions menus accordingly
- Uses accurate size estimates based on actual menu content
- Event menus: 280×200px (recurring) / 180×100px (non-recurring)
- Calendar/generic menus: 180×60px for single items
- Maintains 5px minimum margins from screen edges
- Graceful fallback to original positioning if viewport detection fails

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

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

frontend/src/components/context_menu.rs

@@ -20,9 +20,45 @@ pub fn context_menu(props: &ContextMenuProps) -> Html {
         return html! {};
     }
 
+    // Smart positioning to keep menu within viewport
+    let (x, y) = {
+        let mut x = props.x;
+        let mut y = props.y;
+        
+        // Try to get actual viewport dimensions
+        if let Some(window) = web_sys::window() {
+            if let (Ok(width), Ok(height)) = (window.inner_width(), window.inner_height()) {
+                if let (Some(w), Some(h)) = (width.as_f64(), height.as_f64()) {
+                    let viewport_width = w as i32;
+                    let viewport_height = h as i32;
+                    
+                    // Generic context menu: "Delete Calendar" 
+                    let menu_width = 180; // "Delete Calendar" text + padding
+                    let menu_height = 60; // Single item + padding + margins
+                    
+                    // Adjust horizontally if too close to right edge
+                    if x + menu_width > viewport_width - 10 {
+                        x = x.saturating_sub(menu_width);
+                    }
+                    
+                    // Adjust vertically if too close to bottom edge
+                    if y + menu_height > viewport_height - 10 {
+                        y = y.saturating_sub(menu_height);
+                    }
+                    
+                    // Ensure minimum margins from edges
+                    x = x.max(5);
+                    y = y.max(5);
+                }
+            }
+        }
+        
+        (x, y)
+    };
+
     let style = format!(
         "position: fixed; left: {}px; top: {}px; z-index: 1001;",
-        props.x, props.y
+        x, y
     );
 
     let on_delete_click = {