fix(build): Containerfile builds again — relative module imports, all four pages and shared assets in the image

frontend/index.html

@@ -113,10 +113,9 @@
   </footer>
 
   <script src="assets/markdown.js"></script>
-  <!-- Phase 19: shared header module (whoami caching, Sign in/out,
-       Sources-link toggle, sign-out binding) loads before the page
-       script, which calls initSharedHeader() at boot. -->
-  <script type="module" src="/assets/header.js"></script>
+  <!-- Phase 19: the shared header module loads through the page script's
+       own `import "./header.js"` — a hoisted import that is evaluated
+       before the page script body calls initSharedHeader() at boot. -->
   <script type="module" src="/assets/app.js"></script>
 </body>
 </html>