feat: 管理器支持深浅主题切换，发布 exe

- 新增 LightTheme.xaml 浅色主题资源
- 标题栏添加主题切换按钮（☀️/🌙）
- Manager 支持深浅主题互换
- 发布 Manager.exe (292KB) 和 Installer.exe (171KB)
- 插件安装包 Better-Seewo.2.0.0.exe (11.6MB)

Better-EN5/Services/Conversion/ConversionEngine.cs

@@ -0,0 +1,24 @@
+using System;
+
+namespace BetterEN5.Services.Conversion;
+
+public class ConversionEngine
+{
+    private readonly PptxImporter _importer = new();
+    private readonly EnbxWriter _writer = new();
+
+    public bool Convert(string pptxPath, string enbxPath)
+    {
+        try
+        {
+            var doc = _importer.Import(pptxPath);
+            _writer.Write(doc, enbxPath);
+            return true;
+        }
+        catch (Exception ex)
+        {
+            Console.Error.WriteLine($"Conversion failed: {ex}");
+            return false;
+        }
+    }
+}