fix: add base styles for month/year select dropdowns

Add styling for .react-datepicker__month-select, .react-datepicker__year-select,
and .react-datepicker__month-year-select elements which were previously unstyled.

The new styles use:
- transparent background to inherit from parent (works with dark/light themes)
- color: inherit to match the parent's text color
- consistent border, border-radius, and font styling
- focus-visible outline for accessibility

This fixes issues where the native <select> elements would display with
browser default styling, which looked bad especially in dark mode themes.

Fixes #5786

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: martijnrusschen

src/stylesheets/datepicker.scss

@@ -149,6 +149,23 @@
   margin: 0 15px;
 }
 
+.react-datepicker__month-select,
+.react-datepicker__year-select,
+.react-datepicker__month-year-select {
+  background-color: transparent;
+  border: 1px solid $datepicker__border-color;
+  border-radius: $datepicker__border-radius;
+  color: inherit;
+  cursor: pointer;
+  font-family: inherit;
+  font-size: inherit;
+  padding: 2px;
+
+  &:focus-visible {
+    outline: auto 1px;
+  }
+}
+
 .react-datepicker__current-month,
 .react-datepicker-time__header,
 .react-datepicker-year-header {