Implement drag-to-create event functionality in week view

Add interactive drag-to-create event functionality that allows users to click and drag in empty spaces of the week view to create new events. Features include:

- Mouse event handlers for drag interaction (mousedown, mousemove, mouseup)
- Real-time temporary event box display with visual feedback during drag
- Proper coordinate calculation using layer_y() for accurate time positioning
- Minimum 15-minute event duration enforcement
- Integration with event creation modal via callback with pre-filled start/end times
- CSS pointer-events optimizations to prevent child element interference
- Time-to-pixel and pixel-to-time conversion functions for accurate positioning

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

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

styles.css

@@ -582,11 +582,13 @@ body {
     height: 60px;
     border-bottom: 1px solid #f0f0f0;
     position: relative;
+    pointer-events: none; /* Don't capture mouse events */
 }
 
 .time-slot-half {
     height: 30px;
     border-bottom: 1px dotted #f5f5f5;
+    pointer-events: none; /* Don't capture mouse events */
 }
 
 .time-slot-half:last-child {
@@ -597,6 +599,7 @@ body {
     height: 60px; /* Match the final time label height */
     border-bottom: 2px solid #e9ecef; /* Strong border to match final boundary */
     background: rgba(0,0,0,0.02); /* Slightly different background to indicate boundary */
+    pointer-events: none; /* Don't capture mouse events */
 }
 
 /* Events Container */
@@ -606,7 +609,7 @@ body {
     left: 0;
     right: 0;
     bottom: 0;
-    pointer-events: none;
+    pointer-events: none; /* Container doesn't capture, but children (events) do */
 }
 
 /* Week Events */
@@ -644,6 +647,27 @@ body {
     border-color: #ff9800;
 }
 
+/* Temporary event box during drag creation */
+.temp-event-box {
+    position: absolute;
+    left: 4px;
+    right: 4px;
+    background: rgba(59, 130, 246, 0.3);
+    border: 2px dashed rgba(59, 130, 246, 0.8);
+    border-radius: 4px;
+    color: rgba(59, 130, 246, 0.9);
+    font-size: 0.75rem;
+    font-weight: 600;
+    padding: 4px 6px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    pointer-events: none;
+    z-index: 6; /* Higher than events */
+    text-align: center;
+    user-select: none;
+}
+
 .week-event .event-title {
     font-weight: 600;
     margin-bottom: 2px;