Remove FluentAvaloniaUI package and replace with native Avalonia controls

[Copilot]

Replace all FluentAvaloniaUI usages with native Avalonia equivalents, removing the third-party dependency.

Changes

• Theme: FluentAvaloniaTheme → native FluentTheme; added explicit Avalonia.Themes.Fluent package reference
• FontIcon: Replaced ui:FontIcon with TextBlock in MainWindow.axaml and EditorControl.axaml.cs context menu (using Text property instead of Glyph)
• ContentDialog: Rewrote ContentDialogHelper to delegate to the existing MessageBox wrapper (MessageBox.Avalonia) while preserving the ContentDialogChanged callback for caret timer control
• SymbolThemeFontFamily: Defined as an app-level resource in App.axaml since it was previously provided by FluentAvaloniaUI

<!-- Before -->
<sty:FluentAvaloniaTheme PreferSystemTheme="True" PreferUserAccentColor="True" />
<ui:FontIcon FontFamily="{DynamicResource SymbolThemeFontFamily}" Glyph="&#xE711;" />

<!-- After -->
<FluentTheme />
<TextBlock FontFamily="{DynamicResource SymbolThemeFontFamily}" Text="&#xE711;" />

MenuFlyout, MenuItem, and Separator in the context menu required no changes — they resolve to native Avalonia.Controls types once the FluentAvalonia using directive is removed.