fix: Share dialog alignment, z-index, access menu colors, shared section

- Dialog right edge aligns with Share button area (right: 72px)
- z-index raised to 999 to prevent being hidden by other layers
- Max-height reduced to 65vh to fit viewport
- Access menu: !important dark background, proper option styling
- Sidebar Shared section now renders shared_pages dynamically
- board.py: shared_pages added to _sidebar_data() return

app/templates/base.html

@@ -161,7 +161,14 @@
         </div>
       </div>
       <div class="sidebar-section-items" x-show="sectionsOpen.shared" x-transition>
-        <ul class="sidebar-items"><li class="sidebar-item empty-hint"><span class="page-icon">👥</span><span class="page-name text-dim">No shared pages</span></li></ul>
+        <ul class="sidebar-items" data-section="shared">
+          {% for page in shared_pages %}
+            {{ render_tree_item(page) }}
+          {% endfor %}
+          {% if not shared_pages %}
+          <li class="sidebar-item empty-hint"><span class="page-icon">👥</span><span class="page-name text-dim">No shared pages</span></li>
+          {% endif %}
+        </ul>
       </div>
     </div>