Responsive app shell, routes and navigation (#38)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>
Co-committed-by: dev_agent <dev_agent@cubecraftlabs.com>

web/src/pages/PlaceholderPage.tsx

@@ -0,0 +1,9 @@
+export type PlaceholderPageProps = {
+  // The route's screen name, shown as the page heading.
+  title: string
+}
+
+// Stands in for a screen until its feature is built.
+export function PlaceholderPage({ title }: PlaceholderPageProps) {
+  return <h1>{title}</h1>
+}