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

frontend/src/js/lib/xterm-addon-web-links.min.js

@@ -0,0 +1 @@
+// Placeholder — replaced with the real xterm-addon-web-links library after `npm install`.