refactor: src-tauri 重命名为 gui

Tauri CLI 自动检测 workspace 成员，构建不受影响。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

gui/capabilities/default.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "enables the default permissions",
+  "windows": [
+    "main"
+  ],
+  "permissions": [
+    "core:default",
+    "dialog:default"
+  ]
+}