fix: remove duplicate topbar on page editor, position Get Started at bottom

- base.html topbar wrapped in {% block topbar %} for override
- page_editor.html overrides topbar block with empty content (uses its own)
- Get Started toolbar: fixed to bottom center with gradient fade
- No more double Share/Link/Star buttons

app/templates/base.html

@@ -234,6 +234,7 @@
   <div class="main-wrapper">
 
     <!-- Topbar -->
+    {% block topbar %}
     <header class="topbar">
       <div class="topbar-left">
         <div class="topbar-breadcrumb">
@@ -271,6 +272,7 @@
         </button>
       </div>
     </header>
+    {% endblock %}
 
     <!-- Content -->
     <div class="content-area" id="main-content">