Migrate codebase to Avalonia 12 APIs

Apply Avalonia 12 migration changes: replace SystemDecorations with WindowDecorations and remove ExtendClientAreaChromeHints/ExtendClientAreaTitleBarHeightHint usages; update BindingPlugins removal logic (no-op); switch clipboard usage to ClipboardExtensions.SetTextAsync; update Bitmap.CopyPixels calls to the new signature. Replace TextBox.Watermark with PlaceholderText, convert NumberBox styles to FANumberBox and adjust templates, change Checked/Unchecked handlers to IsCheckedChanged, and adapt FluentIcons usages (SymbolIconSource -> FASymbol/FAFont/FluentIcon equivalents). Fix MainWindow partial classes to inherit Window and correct missing variables/fields/usings. Add migration docs/specs/tasks under .trae and include a small TestFluentIcons project for icon testing.

test_fluenticons.cs

@@ -0,0 +1,11 @@
+using System;
+using FluentIcons.Avalonia;
+
+class Test
+{
+    static void Main()
+    {
+        // 尝试创建 SymbolIconSource
+        var source = new SymbolIconSource();
+    }
+}