fix: set fixed height on calendar grid

Add content-start alignment and fixed 192px height to the
calendar grid to prevent layout shift when switching between
months with different numbers of weeks. This ensures consistent
spacing and prevents the calendar from changing size based on
the month being displayed.

apps/tauri/src/lib/components/DateTimePicker.svelte

@@ -121,7 +121,7 @@
     </div>
 
     <!-- Calendar grid -->
-    <div class="grid grid-cols-7 px-4 pb-2">
+    <div class="grid grid-cols-7 content-start px-4 pb-2" style="height: 192px;">
       {#each calendarCells as day}
         {#if day === null}
           <div></div>