feat: mobile search bar in header — tap icon to search Navidrome library

Search icon added to mobile header. Tapping opens an inline search bar
below the header. Typing searches artists/albums/songs via Navidrome
search3 API with 300ms debounce. X button closes and returns to Home.

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

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v25';
+const CACHE = 'ns-v26';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];