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.

svelte.config.js

@@ -1,7 +1,7 @@
-import adapter from '@sveltejs/adapter-netlify';
 import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
 // you don't need to do this if you're using generateSW strategy in your app
 import { generateSW } from './pwa.mjs';
+import { adapter } from './adapter.mjs';
 
 /** @type {import('@sveltejs/kit').Config} */
 const config = {
@@ -10,16 +10,8 @@ const config = {
 	preprocess: vitePreprocess(),
 
 	kit: {
-		adapter: adapter({
-			// if true, will create a Netlify Edge Function rather
-			// than using standard Node-based functions
-			edge: false,
-
-			// if true, will split your app into multiple functions
-			// instead of creating a single one for the entire app.
-			// if `edge` is true, this option cannot be used
-			split: false
-		}),
+		// Netlify by default, or the node adapter when NODE_ADAPTER=true. See adapter.mjs.
+		adapter,
 		serviceWorker: {
 			register: true
 		},