Fixes: #12368

The FixYear parameter was not being used when displaying the year header in month view or initializing the calendar, causing the picker to show the current year instead of the fixed year.

Changes:

• GetCalendarYear(): Return FixYear.Value when set, before falling back to date-based calculation
• GetCalendarStartOfMonth(): Include HighlightedDate in fallback chain (which respects FixYear during initialization)

Example:

<!-- Now correctly displays and constrains to year 1900 -->
<MudDatePicker FixYear="1900" />

The fix ensures that when FixYear is specified (past or future), the DatePicker consistently displays that year in the header and renders the correct month's days without hiding them incorrectly.

Checklist:

• I've read the contribution guidelines
• My code follows the style of this project
• I've added or updated relevant unit tests