fix(editor): placeholder titre "New Page" + aides de bloc au focus seul

- Titre : data-placeholder="New Page" + bascule .empty a l'input (mecanisme des
  blocs). Le placeholder s'affiche en gris pale quand le titre est vide, y compris
  apres effacement. save() stocke un titre vide (plus de "New page" force), la
  sidebar garde son fallback, duplicatePage retombe sur "New Page".
- Blocs : les placeholders ("Press 'space' for AI...", "Heading 1", "List"...)
  passent sous :focus-within -> ne s'affichent que quand le bloc est focalise.
  Idem pour "Type code..." du bloc code.

v5.1.6

app/templates/_page_editor_scripts.html

@@ -700,6 +700,8 @@
           window.E=this;
           this.insertTable=function(){this.sync();var nb={id:genId(),type:'table',content:'',rows:_tblNew(3,3)};this.blocks.push(nb);this.dirty=true;this.autoSave();this.render();setTimeout(function(){_tblFocus(nb.id,0,0);},60);};
           SM.init();_tblInitDelegate();this.render();
+          const tEl=document.getElementById('_titleEl');
+          if(tEl){const syncT=()=>{tEl.classList.toggle('empty',!(tEl.textContent||'').trim());};syncT();tEl.addEventListener('input',syncT);tEl.addEventListener('blur',syncT);}
           setTimeout(()=>{this.focusBlock();const ct=document.getElementById('_blocksCt');if(!ct)return;
             ct.addEventListener('keydown',e=>this.onKd(e));
             ct.addEventListener('paste',e=>{e.preventDefault();document.execCommand('insertText',false,(e.clipboardData||window.clipboardData).getData('text/plain'));this.dirty=true;this.autoSave();});
@@ -977,7 +979,7 @@ applyAIBlocks(text){
             doDownload();
           }
         },
-        duplicatePage(){this.moreOpen=false;const csrf=document.cookie.match(/csrf_token=([^;]+)/);fetch(`/board/api/pages?title=${encodeURIComponent(this.pageTitle+' copy')}&section=Private&project=${encodeURIComponent('{{ workspace_key }}')}`,{method:'POST',headers:{'X-CSRF-Token':csrf?csrf[1]:''}}).then(r=>r.json()).then(d=>{window.location.href=`/pages/${d.id}`;}).catch(()=>{window.showToast('Duplicate failed','error');});},
+        duplicatePage(){this.moreOpen=false;const csrf=document.cookie.match(/csrf_token=([^;]+)/);const t=(this.pageTitle||'').trim()||'New Page';fetch(`/board/api/pages?title=${encodeURIComponent(t+' copy')}&section=Private&project=${encodeURIComponent('{{ workspace_key }}')}`,{method:'POST',headers:{'X-CSRF-Token':csrf?csrf[1]:''}}).then(r=>r.json()).then(d=>{window.location.href=`/pages/${d.id}`;}).catch(()=>{window.showToast('Duplicate failed','error');});},
         movePage(){
           this.moreOpen=false;
           this.moveOpen = true;
@@ -1276,7 +1278,7 @@ applyAIBlocks(text){
           if(this.fileData)return;
           if(this.saving){ if(cb) cb(); return; }
           this.sync();
-          const t=document.getElementById('_titleEl');if(t)this.pageTitle=t.textContent?.trim()||'New page';
+          const t=document.getElementById('_titleEl');if(t)this.pageTitle=t.textContent?.trim()||'';
           this.saving=true;
           const csrf=document.cookie.match(/csrf_token=([^;]+)/);
           fetch(`/board/api/pages/${this.pid}/blocks`,{method:'POST',headers:{'Content-Type':'application/json','X-CSRF-Token':csrf?csrf[1]:''},body:JSON.stringify({title:this.pageTitle,blocks:this.blocks.map(b=>{const c={id:b.id,type:b.type,content:b.content};['checked','expanded','language','icon','src','alt','style','embed_type','collection_id','dbs','file_name','file_size','file_mime','children','rows','align','has_header','colsW','meeting'].forEach(k=>{if(b[k]!=null)c[k]=b[k];});return c;})})})