feat: GUI 端新增解码功能 — 选择图片解码 QR 码

- 新增 decode_qr Tauri command（接收图片字节，返回解码文本）
- ExportPanel 新增「选择图片解码」按钮 + 解码结果展示
- fs:allow-read-file 权限，支持 /c/Users/33644 和 C:\Users\33644\AppData\Local\Temp 路径

gui/src-frontend/src/components/ExportPanel.tsx

@@ -2,8 +2,8 @@ import { useState } from 'react';
 import { useQrState } from '../store/qrContext';
 import { invoke } from '@tauri-apps/api/core';
 import { writeText } from '@tauri-apps/plugin-clipboard-manager';
-import { save } from '@tauri-apps/plugin-dialog';
-import { writeFile } from '@tauri-apps/plugin-fs';
+import { open, save } from '@tauri-apps/plugin-dialog';
+import { readFile, writeFile } from '@tauri-apps/plugin-fs';
 import type { QrConfig } from '../types';
 import { buildEncodedText } from '../utils/qrText';
 
@@ -11,6 +11,30 @@ export default function ExportPanel() {
   const { state, dispatch } = useQrState();
   const [exporting, setExporting] = useState(false);
   const [errorMsg, setErrorMsg] = useState<string | null>(null);
+  const [decodedText, setDecodedText] = useState<string | null>(null);
+  const [decoding, setDecoding] = useState(false);
+
+  const handleDecode = async () => {
+    setDecoding(true);
+    setErrorMsg(null);
+    setDecodedText(null);
+    try {
+      const filePath = await open({
+        filters: [{ name: '图片文件', extensions: ['png', 'jpg', 'jpeg', 'webp', 'bmp'] }],
+        multiple: false,
+      });
+      if (!filePath) {
+        setDecoding(false);
+        return;
+      }
+      const bytes = await readFile(filePath);
+      const text: string = await invoke('decode_qr', { imageBytes: Array.from(bytes) });
+      setDecodedText(text);
+    } catch (e) {
+      setErrorMsg(`解码失败: ${e}`);
+    }
+    setDecoding(false);
+  };
 
   const handleCopySvg = async () => {
     if (!state.preview?.svg) return;
@@ -139,6 +163,25 @@ export default function ExportPanel() {
       >
         导出 SVG
       </button>
+
+      {/* 解码区 */}
+      <div className="mt-3 pt-3 border-t border-gray-200 dark:border-gray-700">
+        <div className="text-xs font-semibold text-gray-400 uppercase tracking-wider mb-2">
+          解码
+        </div>
+        <button
+          onClick={handleDecode}
+          disabled={decoding}
+          className="w-full py-2 rounded-lg bg-amber-500 text-white text-sm font-medium hover:bg-amber-600 disabled:opacity-40 transition-all"
+        >
+          {decoding ? '解码中...' : '选择图片解码'}
+        </button>
+        {decodedText && (
+          <div className="mt-2 p-2 bg-green-50 dark:bg-green-900/20 rounded-lg text-xs text-green-700 dark:text-green-300 break-all max-h-24 overflow-auto">
+            {decodedText}
+          </div>
+        )}
+      </div>
     </div>
   );
 }