fix: wire up frontend build pipeline for Wails v2

- embed.go: embed index.html + all:src directly instead of all:dist,
  so `go build` works without a prior npm build step
- index.html: add missing xterm.css vendor stylesheet link
- build.js: add buildDist() to assemble dist/ with index.html and the
  full src/ tree; update required-file paths to match dist/ layout
- Add placeholder xterm lib stubs in src/js/lib/ and src/css/lib/ so
  the app can start before `npm install` is run
- .gitignore: stop ignoring the lib placeholder files so they are
  available on a fresh clone

.gitignore

@@ -5,8 +5,6 @@ dist/
 # Dependencies
 vendor/
 frontend/node_modules/
-frontend/src/js/lib/
-frontend/src/css/lib/
 
 # IDE
 .idea/