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/index.html

@@ -141,8 +141,6 @@
                     <polyline points="10 9 9 9 8 9"/>
                 </svg>
             </button>
-            <input type="file" id="fileInput" accept="image/*" multiple style="display:none;">
-            <input type="file" id="textFileInput" accept=".txt,.md,.log,.csv,.py,.pyw,.js,.mjs,.cjs,.ts,.tsx,.jsx,.java,.c,.h,.cpp,.cc,.hpp,.go,.rs,.rb,.php,.sh,.bash,.zsh,.sql,.html,.htm,.css,.json,.jsonl,.xml,.svg,.yaml,.yml,.toml,.ini,.cfg,.conf,.swift,.kt,.kts,.scala,.lua,.r,.m,.cs,.fs,.ex,.exs,.erl,.hs,.clj,.lisp,.diff,.patch,Dockerfile,Makefile,.rst,.tsv,.vb,.pl,.mm,.pyi,.fsharp,.makefile,.dockerfile" multiple style="display:none;">
             <textarea class="chat-input" id="chatInput" placeholder="输入消息..." rows="1"></textarea>
             <label class="think-toggle" id="thinkToggleWrap" title="Think 模式未可用">
                 <input type="checkbox" id="toggleThink" disabled>
@@ -264,7 +262,6 @@
                 <div style="display:flex;gap:8px;flex-wrap:wrap;">
                     <button class="btn btn-outline" id="btnExportAll">📦 导出全部会话</button>
                     <button class="btn btn-outline" id="btnImportSessions">📥 导入会话</button>
-                    <input type="file" id="importFileInput" accept=".metona" style="display:none;">
                     <button class="btn btn-danger" id="btnClearAllHistory">清空所有历史</button>
                 </div>
                 <p class="text-muted" style="margin-top:6px;font-size:11px;">导出格式为 JSON，可备份后在其他设备导入恢复。</p>
@@ -344,7 +341,6 @@
                 <div class="kb-main">
                     <div class="kb-toolbar">
                         <button class="btn btn-sm btn-outline" id="btnUploadDoc">📄 上传文档</button>
-                        <input type="file" id="kbFileInput" accept=".txt,.md,.py,.js,.ts,.java,.go,.rs,.cpp,.c,.h,.hpp,.rb,.php,.sh,.html,.css,.json,.yaml,.yml,.toml,.xml,.sql,.csv" multiple style="display:none;">
                         <span id="kbProgress" style="display:none;" class="kb-progress">
                             <span class="spinner"></span>
                             <span id="kbProgressText">处理中...</span>