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

static/css/app.css

@@ -2162,8 +2162,12 @@ input, select, textarea { font-family: inherit; color: inherit; }
 
 /* ═══════════ Get Started Toolbar (bottom, pill buttons) ═══════════ */
 .get-started-toolbar {
+  position: fixed; bottom: 0; left: 50%; transform: translateX(-50%);
   display: flex; align-items: center; gap: 8px;
-  padding: 16px 0 8px; flex-wrap: wrap;
+  padding: 12px 0 16px; flex-wrap: wrap;
+  justify-content: center; z-index: 10;
+  background: linear-gradient(to top, var(--bg-primary) 60%, transparent);
+  width: 100%; max-width: 900px;
 }
 .gs-label {
   font-size: 13px; color: var(--text-dim); margin-right: 4px;