Now Playing card on the Music tab; Signal alerts (chime, desktop notification, unread count in tab title and app icon)

- Music: a card with art, seek bar and controls above the albums, artists and playlists
- Signal: chime, optional desktop notification when the app is in the background, (n) in the tab title, app badge; Settings -> Signal alerts (per device)
- README, player test stand-in updated

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tools/player-test.js

@@ -7,8 +7,8 @@ class FakeAudio{constructor(){this.paused=true;this.currentTime=0;this.duration=
  addEventListener(e,f){(this._l[e]=this._l[e]||[]).push(f)} fire(e){(this._l[e]||[]).forEach(f=>f())}
  play(){this.paused=false;this.fire('play');return Promise.resolve()} pause(){if(!this.paused){this.paused=true;this.fire('pause')}} getAttribute(){return this.src||null} load(){}}
 const toasts=[];const store={d:{},get(k,d){return this.d[k]??d},set(k,v){this.d[k]=v},del(k){delete this.d[k]}};
-const el=()=>({classList:{toggle(){},add(){}},setAttribute(){},dataset:{},style:{},querySelector:()=>el(),querySelectorAll:()=>[],set innerHTML(v){},set textContent(v){}});
-const ctx={Audio:FakeAudio,store,toast:m=>toasts.push(m),px:{art:()=>'x'},icon:()=>'',esc:s=>s,fmtDur:()=>'',$:()=>el(),document:{querySelectorAll:()=>[]},
+const el=()=>({remove(){},classList:{toggle(){},add(){}},setAttribute(){},dataset:{},style:{},querySelector:()=>el(),querySelectorAll:()=>[],set innerHTML(v){},set textContent(v){}});
+const ctx={Audio:FakeAudio,store,toast:m=>toasts.push(m),px:{art:()=>'x'},icon:()=>'',esc:s=>s,fmtDur:()=>'',$:()=>el(),document:{querySelectorAll:()=>[],querySelector:()=>null},
  navigator:{onLine:true},addEventListener:(e,f)=>{listeners[e]=f},setTimeout,clearTimeout,fetch:()=>new Promise(()=>{}),URL:{createObjectURL:()=>'blob:x',revokeObjectURL(){}},
  Image:class{},MediaMetadata:class{},openSheet:()=>({}),closeSheet(){},actionSheet(){},Response,URLSearchParams,AbortController,Promise,Set,Map,Date,Math,saveStateHook:0,location:{hash:''}};
 vm.createContext(ctx); vm.runInContext(code+";this.pl=pl;this.next=next;this.prev=prev;this.setShuffle=setShuffle;this.setRepeat=setRepeat;this.playQueue=playQueue;this.advance=advance;this.cur=cur;this.recover=recover;this.srcOf=srcOf;",ctx);