Send and store base_hash for optimistic conflict detection

src/components/PageEditor.svelte

@@ -14,6 +14,7 @@
   let saveStatus = '';
   let savedUrl = '';
   let errorMsg = '';
+  let baseHash = '';
   let uploading = false;
   let dragOver = false;
   let fileInput: HTMLInputElement;
@@ -36,6 +37,7 @@
     saveStatus = '';
     savedUrl = '';
     errorMsg = '';
+    baseHash = '';
   }
 
   function onSectionChange() {
@@ -63,6 +65,7 @@
       if (!r.ok) throw new Error(`HTTP ${r.status}`);
       const data = await r.json();
       content = data.content;
+      baseHash = data.base_hash ?? '';
     } catch (e) {
       errorMsg = `Errore nel caricamento: ${e}`;
     }
@@ -80,8 +83,9 @@
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
         credentials: 'include',
-        body: JSON.stringify({ content }),
+        body: JSON.stringify({ content, base_hash: baseHash }),
       });
+      if (r.status === 409) { errorMsg = await r.text(); return; }
       if (!r.ok) throw new Error(await r.text());
       saveStatus = 'Costruendo…';
       const rb = await fetch('/rebuild', { method: 'POST', credentials: 'include' });