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/index.html

@@ -5,6 +5,9 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Nexus Terminal</title>
 
+    <!-- Vendor styles -->
+    <link rel="stylesheet" href="/src/css/lib/xterm.css">
+
     <!-- Design system -->
     <link rel="stylesheet" href="/src/css/tokens.css">
     <link rel="stylesheet" href="/src/css/base.css">