Fix calendar layout positioning and overflow handling

- Add position relative and height 100% to calendar component for proper layout
- Add overflow hidden to week events overlay to prevent content spillover

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

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

frontend/styles.css

@@ -1168,6 +1168,8 @@ body {
     flex: 1;
     display: flex;
     flex-direction: column;
+    position: relative;
+    height: 100%;
 }
 
 .calendar-header {
@@ -1509,6 +1511,7 @@ body {
     right: 0;
     bottom: 0;
     pointer-events: none; /* Container doesn't capture, but children (events) do */
+    overflow: hidden;
 }
 
 /* Week Events */