Fix drag interaction issues in week view

- Fix drag-to-create being blocked by existing events
- Add creating-event CSS class that disables pointer events on existing events
- Fix single clicks creating temporary event boxes
- Add mouse button state check to prevent post-mouseup movement being treated as drag
- Ensure temp event boxes only appear during actual drag operations (has_moved=true)

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

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

frontend/styles.css

@@ -802,6 +802,20 @@ body {
     white-space: nowrap;
 }
 
+/* Disable pointer events on existing events when creating a new event */
+.week-day-column.creating-event .week-event {
+    pointer-events: none;
+    opacity: 0.6; /* Visual feedback that events are not interactive */
+}
+
+.week-day-column.creating-event .week-event .event-content {
+    pointer-events: none;
+}
+
+.week-day-column.creating-event .week-event .resize-handle {
+    pointer-events: none;
+}
+
 .week-event:hover {
     filter: brightness(1.1);
     z-index: 4;