refactor(qrCode): 优化二维码解析功能

- 将'二维码转 URL'改为'二维码转文本'
- 将解析结果的文本预览框的 placeholder 置为空
- 去除手动解析二维码功能，只保留自动解析

i18n/locales/en/qrCode.json

@@ -5,8 +5,8 @@
   "generateMode": "Generate QR Code",
   "parseMode": "Parse QR Code",
 
-  "urlToQr": "URL to QR Code",
-  "qrToUrl": "QR Code to URL",
+  "urlToQr": "Text to QR Code",
+  "qrToUrl": "QR Code to Text",
 
   "urlInputLabel": "Enter URL or Text",
   "urlInputPlaceholder": "Enter URL or text content, QR code will be generated automatically",