fix: playlist cover upload — POST /api/playlist/{id}/image FormData with JWT

Confirmed endpoint pattern from Navidrome API contract (artist/:id/image).
FormData field 'image' + X-Nd-Authorization Bearer token. Bump SW ns-v36.

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

app.js

@@ -505,13 +505,12 @@ async function editPlCoverSelected(input){
   const preview=URL.createObjectURL(file);
   document.getElementById('edit-pl-cover').innerHTML=`<img src="${preview}" style="width:100%;height:100%;object-fit:cover">`;
   try{
-    const imageData=await new Promise((res,rej)=>{const r=new FileReader();r.onload=()=>res(r.result);r.onerror=rej;r.readAsDataURL(file);});
     const token=await ndAuth();
-    const pl=_currentPlaylist;
-    const res=await fetch(`${cfg.url}/api/playlist/${pl.id}`,{
-      method:'PUT',
-      headers:{'X-Nd-Authorization':'Bearer '+token,'Content-Type':'application/json'},
-      body:JSON.stringify({name:pl.name,comment:pl.comment||'',public:!!pl.public,imageData})
+    const form=new FormData();form.append('image',file,file.name);
+    const res=await fetch(`${cfg.url}/api/playlist/${_currentPlaylist.id}/image`,{
+      method:'POST',
+      headers:{'X-Nd-Authorization':'Bearer '+token},
+      body:form
     });
     if(!res.ok)throw new Error(res.status);
     toast('Cover aktualisiert');