chore: init v2 project scaffold

- Add electron-vite + Vue 3 + TypeScript build config
- Add .gitignore with legacy/ exclusion
- Add AGENTS.md with architecture & dev commands
- Update README.md

.gitignore

@@ -0,0 +1,18 @@
+node_modules/
+out/
+dist/
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Editor
+.idea
+.vscode
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+
+# Legacy codebase — archived Vue 2 version, not part of v2 build
+legacy/