Fix calendar height to maintain size when time range narrows

Use fixed 600px height instead of auto so that when the visible
time range compresses to match available slots, the calendar
maintains the same visual height and gives more space to events.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeremymanning

js/calendar.js

@@ -30,7 +30,7 @@ var CalendarUI = (function () {
       slotMaxTime: DEFAULT_MAX_TIME,
       slotDuration: minutesToSlotDuration(durationMinutes),
       timeZone: 'local',
-      height: 'auto',
+      height: 600,
       nowIndicator: true,
       selectable: false,
       editable: false,