Add Calendar component with date selection and navigation

[Copilot]

Implements Calendar component for migrating ASP.NET Web Forms Calendar controls to Blazor with minimal markup changes.

Implementation

Core Component

• Table-based rendering matching Web Forms HTML output
• Selection modes: None, Day, DayWeek, DayWeekMonth
• Month/year navigation with customizable text
• Event callbacks: OnSelectionChanged, OnDayRender, OnVisibleMonthChanged

Display & Formatting

• Day name formats: Full, Short, FirstLetter, FirstTwoLetters, Shortest
• Title formats: Month, MonthYear
• Configurable first day of week, cell padding/spacing
• Toggle title bar, day headers, grid lines, navigation

Styling

• CSS class-based styling for day types: today, selected, weekend, other month
• Compatible with existing Web Forms stylesheets
• Custom TitleStyleCss, DayStyleCss, SelectedDayStyleCss, etc.

Code Safety

• Culture-safe substring operations for day name formatting
• Synchronous OnDayRender event handling to allow day property modification before rendering

Usage

<Calendar @bind-SelectedDate="selectedDate" 
          SelectionMode="DayWeek"
          ShowGridLines="true"
          DayNameFormat="Short"
          OnSelectionChanged="HandleSelection" />

@code {
    private DateTime selectedDate = DateTime.Today;
    
    private void HandleSelection()
    {
        // selectedDate contains the selected date
    }
}

Testing & Documentation

• 19 unit tests covering rendering, selection, navigation, formatting
• Comprehensive documentation in docs/EditorControls/Calendar.md
• Sample page with live examples at /ControlSamples/Calendar

Screenshot

Shows basic calendar, selection modes, custom formatting, and styled variations.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Create Calendar component</issue_title>
<issue_description>We need the Calendar component created with all features from the ASP.NET control</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create Calendar component #332

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[csharpfritz]

@copilot add the individual style objects to this component to mirror the existing control. We should add a marker to indicate we don't recommend their use