fix(pwa): honour the adapter flag and precache an offline entry point

Three defects that together meant `npm run test:build` could not pass in any of
its four variants:

- svelte.config.js constructed adapter-netlify inline and never used the
  `adapter` export from adapter.mjs, so NODE_ADAPTER=true still produced a flat
  build/ directory while the build test expects the node adapter's build/client
  layout. adapter.mjs now returns netlify (the deployment target) or node, and
  svelte.config.js consumes it.

- No route is prerendered, so workbox's glob found no HTML document and a
  generateSW build precached nothing navigable: the app had no offline support
  in that mode at all. Adds the root entry and a navigation fallback, matching
  what prompt-sw.ts already did by hand for injectManifest builds.

- The build scripts used by the tests skipped the tailwind step that `build`
  runs, so static/output.css was never generated on a clean checkout and the
  app under test had no stylesheet.

The offline entry point assertion now also accepts the unquoted object key that
prompt-sw.ts's own precache call survives minification as.

vite.config.ts

@@ -51,6 +51,11 @@ export default defineConfig({
 			workbox: {
 				globPatterns: ['client/**/*.{js,css,ico,png,svg,webp,woff,woff2,webmanifest}'],
 				globIgnores: ['**/sprites/**', '**/sprites-small/**'],
+				// No route is prerendered, so globbing finds no HTML document and a generateSW
+				// build would precache nothing navigable - i.e. no offline support at all.
+				// prompt-sw.ts does the equivalent with `precache([{ url: '/' }])`.
+				additionalManifestEntries: [{ url: '/', revision: null }],
+				navigateFallback: '/',
 				runtimeCaching: [
 					{
 						urlPattern: ({ request }) => request.destination === 'image',