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.

adapter.mjs

@@ -1,7 +1,20 @@
-import process from 'node:process'
+import process from 'node:process';
 import AdapterNode from '@sveltejs/adapter-node';
-import AdpaterStatic from '@sveltejs/adapter-static';
+import AdapterNetlify from '@sveltejs/adapter-netlify';
 
-export const nodeAdapter = process.env.NODE_ADAPTER === 'true'
+export const nodeAdapter = process.env.NODE_ADAPTER === 'true';
 
-export const adapter = nodeAdapter ? AdapterNode() : AdpaterStatic()
+// Netlify is the deployment target; the node adapter exists so the service worker
+// build tests can check the `build/client` layout a Node server produces.
+export const adapter = nodeAdapter
+	? AdapterNode()
+	: AdapterNetlify({
+			// 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
+		});