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.

tests/build/build.test.ts

@@ -29,7 +29,9 @@ describe(`test-build: ${nodeAdapter ? 'node' : 'static'} adapter`, () => {
 			match && match.length === 1,
 			'missing manifest.webmanifest in sw precache manifest'
 		).toBeTruthy();
-		match = swContent.match(/"url":\s*"(?:\/|index\.html)"/);
+		// The generateSW manifest is emitted as JSON ("url": "/"), while prompt-sw.ts's own
+		// `precache([{ url: '/' }])` survives minification as an unquoted key (url:"/").
+		match = swContent.match(/"?url"?:\s*"(?:\/|index\.html)"/);
 		expect(
 			match && match.length === 1,
 			'missing offline entry point in sw precache manifest'