- 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