feat: Add SkiaFontRegistrar to fix CJK/icon-font garbling on Android

[Copilot]

On Android, SKFontManager.Default does not expose CJK or icon-font typefaces. SkiaDrawingContext was falling through to SKTypeface.FromFamilyName() which silently returned the default Latin typeface, rendering Japanese headers as garbled boxes (see screenshot above).

Core fix — KumikoUI.SkiaSharp

New SkiaFontRegistrar static class — a thread-safe registry that bypasses the system font manager entirely for named families:

Method	Ownership
RegisterTypeface(family, typeface)	Caller retains
RegisterTypefaceFromStream(family, stream)	Registrar owns; disposed on Clear()
TryGetTypeface / IsRegistered / Clear	Internal + lifecycle

SkiaDrawingContext changes:

• GetOrCreateTypeface() checks SkiaFontRegistrar first before SKTypeface.FromFamilyName()
• Dispose() skips registered typefaces (externally owned — must not be disposed per-frame)

Sample app

• NotoSansJP-Regular.ttf and MaterialIcons-Regular.ttf added to Resources/Raw/
• MauiProgram.cs registers both at startup via a safe blocking load (pre-message-loop)
• New Custom Fonts tab (CustomFontsPage) demonstrates Japanese column headers and a Material Icons icon-font cell renderer

Usage

// MauiProgram.cs
using var stream = FileSystem.OpenAppPackageFileAsync("NotoSansJP-Regular.ttf")
    .GetAwaiter().GetResult(); // safe — no sync-context before MAUI message loop starts
SkiaFontRegistrar.RegisterTypefaceFromStream("NotoSansJP", stream);

// DataGridStyle
style.HeaderFont = new GridFont("NotoSansJP", 14, bold: true);
style.CellFont   = new GridFont("NotoSansJP", 13);

Icon fonts work identically — set GridFont.Family to the registered name and use Unicode code-point strings as cell values.