Replace ES module import/export with a global namespace pattern to avoid
silent failures in the Wails v2 WKWebView which does not reliably support
<script type="module"> tags.
- Remove all import/export statements from JS files
- Register each module's exports on window.NexusTerminal
- Destructure dependencies from window.NexusTerminal at file top
- Load all scripts in dependency order in index.html as plain <script> tags
- 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
andy