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/embed.go

@@ -3,7 +3,11 @@ package frontend
 
 import "embed"
 
-// Assets contains the built frontend files from the dist directory.
+// Assets contains the frontend files served by the Wails asset server.
+// We embed the root index.html and the entire src/ tree (stylesheets,
+// scripts, vendor libs and static assets) so that `go build` works
+// without a separate npm build step.
 //
-//go:embed all:dist
+//go:embed index.html
+//go:embed all:src
 var Assets embed.FS