Simplify ColorPicker and ColorView via shared abstract base class

[Copilot]

ColorPicker inherited from ColorView and its template was a ~500-line verbatim copy of the ColorView template embedded in a flyout — with a comment literally saying "It MUST always be kept in sync with the ColorView".

Changes

New ColorPickerBase abstract class

• Extracts all ~25 shared StyledProperty definitions (Color, HsvColor, Palette, PaletteColors, SelectedIndex, all Is*Visible flags, etc.) into ColorPickerBase.Properties.cs
• Moves ColorChanged event, OnColorChanged, and all shared property-sync logic (Color↔HsvColor, Palette→PaletteColors, alpha coercion) into ColorPickerBase.cs

ColorView → extends ColorPickerBase

• Retains only template-specific code: PART_HexTextBox, PART_TabControl, ValidateSelection, hex input handlers
• ColorView.Properties.cs emptied (all properties now in base class)

ColorPicker → extends ColorPickerBase (no longer ColorView)

• Retains only Content, ContentTemplate, HorizontalContentAlignment, VerticalContentAlignment
• Adds [TemplatePart("PART_ColorView", typeof(ColorView))]

Simplified ColorPicker templates (both Simple and Fluent)

~500 lines of duplicated XAML reduced to ~99 lines using <ColorView> as the inner flyout element with bindings:

<ColorView x:Name="PART_ColorView"
           Color="{Binding Color, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
           SelectedIndex="{Binding SelectedIndex, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
           ColorModel="{TemplateBinding ColorModel}"
           IsAlphaEnabled="{TemplateBinding IsAlphaEnabled}"
           Palette="{TemplateBinding Palette}"
           ...>
  <ColorView.Resources>
    <CornerRadius x:Key="ColorViewTabBackgroundCornerRadius">0,0,0,0</CornerRadius>
  </ColorView.Resources>
</ColorView>

Color and SelectedIndex are TwoWay so user interactions propagate back to the ColorPicker. All configuration properties use one-way TemplateBinding. The ColorViewTabBackgroundCornerRadius resource is overridden locally per-theme to maintain flyout-appropriate styling.