feat: Monaco editor for StringEditor

- Install monaco-editor + @guolao/vue-monaco-editor
- main.ts: loader.config + MonacoEnvironment worker setup
- StringEditor.vue: replace textarea with Monaco editor
- JSON syntax highlighting, folding, word wrap
- Read-only mode when not editing, auto-detect JSON
- Theme follows app theme (vs-dark/vs)
- Font size follows app settings

package.json

@@ -64,9 +64,11 @@
     }
   },
   "dependencies": {
+    "@guolao/vue-monaco-editor": "^1.6.0",
     "electron-store": "^11.0.2",
     "electron-updater": "^6.8.9",
     "ioredis": "^5.7.0",
+    "monaco-editor": "^0.55.1",
     "pinia": "^3.0.4",
     "ssh2": "^1.17.0"
   },