feat: 文件对话框全部改用 Electron 原生对话框

- input-area.ts: 图片/文件上传改用 dialog.showOpenDialog（通过 IPC）
- settings-modal.ts: 导入改用原生打开对话框，导出改用原生保存对话框
- kb-modal.ts: 知识库文档上传改用原生打开对话框
- ipc.ts/preload.ts/types.d.ts: writeFile 增加 encoding 参数支持二进制写入
- index.html: 移除 4 个无用的 <input type="file"> 元素
- 清理死代码: fileToBase64, fileToText, isMetonaFile

src/renderer/types.d.ts

@@ -198,7 +198,7 @@ export interface MetonaDesktopAPI {
   };
   fs: {
     readFile: (filePath: string) => Promise<{ success: boolean; content?: string; name?: string; error?: string }>;
-    writeFile: (filePath: string, content: string) => Promise<{ success: boolean; error?: string }>;
+    writeFile: (filePath: string, content: string, encoding?: string) => Promise<{ success: boolean; error?: string }>;
   };
   tool: {
     execute: (toolName: string, args: Record<string, unknown>) => Promise<ToolResult>;