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.

TestFluentIcons/Program.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Linq;
+using FluentAvalonia.UI.Controls;
+
+class Test
+{
+    static void Main()
+    {
+        var faSymbols = new System.Collections.Generic.HashSet<string>(Enum.GetNames(typeof(FASymbol)));
+
+        // 从错误信息中提取的图标名称
+        var usedIcons = new[]
+        {
+            "Info", "Color", "Apps", "Code", "Home", "Settings",
+            "WeatherMoon", "Search", "Location", "City", "Warning",
+            "ShieldDismiss", "Shield", "Announcements", "Package",
+            "StatusCircle", "Book", "BranchFork", "ArrowSync",
+            "GlobeArrowForward", "Options", "Store", "Layer",
+            "FolderOpen", "Clock", "Maximize"
+        };
+
+        Console.WriteLine("Checking icon availability in FASymbol:");
+        foreach (var icon in usedIcons.Distinct().OrderBy(i => i))
+        {
+            bool exists = faSymbols.Contains(icon);
+            Console.WriteLine($"  {icon}: {(exists ? "OK" : "MISSING")}");
+        }
+    }
+}