feat(nav): partial HTMX navigation — swap main content without full reload

Intercept internal links and load only .main-wrapper via htmx.ajax, keeping the sidebar/header shell in place. Adds history pushState/popstate handling, sidebar active-state resync and document title update. Moves page script blocks inside #main-content so they run on swap, and makes editor/board scripts idempotent. Routes navigateTo/breadcrumb/library/local-workspace through window.fdNavigate and fixes the duplicated navigateTo definition.

app/templates/library.html

@@ -1303,7 +1303,8 @@ function libraryPage() {
     // ── Item actions ──
     openItem(item) {
       if (!item || !item.id) return;
-      window.location.href = '/pages/' + item.id;
+      if (window.fdNavigate) window.fdNavigate('/pages/' + item.id);
+      else window.location.href = '/pages/' + item.id;
     },
 
     startRename(item) {
@@ -1361,7 +1362,8 @@ function libraryPage() {
               return;
             }
           }
-          window.location.href = '/pages/' + d.id;
+          if (window.fdNavigate) window.fdNavigate('/pages/' + d.id);
+          else window.location.href = '/pages/' + d.id;
         }
       } catch(e) {
         if (window.showToast) window.showToast('Failed to create page', 'error');