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>

app.js

@@ -508,6 +508,32 @@ async function doLocalSearch(q){
 }
 function mkSection(title){const d=document.createElement('div');d.className='search-section';d.innerHTML=`<div class="search-section-title">${title}</div>`;return d}
 
+function toggleMobileSearch(){
+  const bar=document.getElementById('mobile-search-bar');
+  const btn=document.getElementById('mobile-search-btn');
+  const open=bar.style.display==='none'||bar.style.display==='';
+  bar.style.display=open?'block':'none';
+  btn.style.color=open?'var(--accent)':'var(--text2)';
+  if(open){setTimeout(()=>document.getElementById('mobile-search-input').focus(),50);}
+  else{closeMobileSearch();}
+}
+function closeMobileSearch(){
+  const bar=document.getElementById('mobile-search-bar');
+  const inp=document.getElementById('mobile-search-input');
+  const btn=document.getElementById('mobile-search-btn');
+  bar.style.display='none';
+  btn.style.color='var(--text2)';
+  inp.value='';
+  navigate('home');
+}
+let _mobileSearchTimer=null;
+function onMobileSearch(val){
+  clearTimeout(_mobileSearchTimer);
+  if(!val.trim()){navigate('home');return;}
+  navigate('search');
+  _mobileSearchTimer=setTimeout(()=>doLocalSearch(val),300);
+}
+
 // ─── ITUNES API ───
 const ITUNES='https://itunes.apple.com';
 async function itunesSearch(query,entity='musicTrack',limit=25){