Fix bottom nav full-width layout + add backend URL autofill

- Remove max-width/gap/padding from bottom nav so buttons fill full screen width
- Add NS_BACKEND_URL to config.js entrypoint and autofill backend-url field on setup screen

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v12';
+const CACHE = 'ns-v13';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'sw.js'];
 const SKIP_URLS = ['/rest/stream', '/rest/download', '/config.js'];