Implement dynamic 15-minute time grid density and remove final boundary

- Scale time grid height dynamically based on time increment (1530px/2970px)
- Add quarter-mode CSS classes for 15-minute blocks (30px each, same as 30-min blocks)
- Update pixel-to-time conversion functions with 2px:1min scaling in 15-min mode
- Generate correct number of time slots (4 per hour in 15-min mode)
- Remove unnecessary final boundary time label and related CSS
- Fix CSS grid layout by removing malformed CSS syntax
- All time-related containers scale properly between 30-minute and 15-minute modes

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

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

frontend/styles/default.css

@@ -648,6 +648,16 @@ body {
     border-bottom: none;
 }
 
+.time-slot-quarter {
+    height: 30px;
+    border-bottom: 1px dotted var(--calendar-border-light, #f8f8f8);
+    pointer-events: none; /* Don't capture mouse events */
+}
+
+.time-slot-quarter:last-child {
+    border-bottom: none;
+}
+
 .time-slot.boundary-slot {
     height: 60px; /* Match the final time label height */
     border-bottom: 2px solid #e9ecef; /* Strong border to match final boundary */