feat(editor): align selection popup and all editor control elements styling with Reader (#81)

## Summary of Changes
This pull request aligns all major interactive editor control elements in the Milkdown Crepe editor with the premium `SelectionAiPanel` / `IntelligenceToolbar` glassmorphism design.

### Changes:
1. **Selection Bubble Menu Unification:** Relocated the selection menu overrides from `Creator.razor.css` to `app.css` to resolve scoping bugs. Themed to match the Reader's selection popup 1:1.
2. **Editor Controls Theming:** Themed table cell drag handles, table actions popups, line insertion handles & add buttons, Notion-style paragraph drag handles, and slash commands menus with glassmorphic backgrounds, perimeter borders, hover transitions, and active accent states.
3. **Visibility Lifecycle Fixes:** Excluded `.cell-handle` and `.milkdown-block-handle` from explicit `display: none !important` rules when hidden, preserving their dimensions for correct JS positioning calculations and preventing handles from jumping/sliding.
4. **Table Margin Clipping Fix:** Set `overflow: visible !important` on `.tableWrapper` to allow table controls to draw cleanly into the editor canvas's padding zone without boundary clipping.

Resolves #82.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #81
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Layout/MainHubLayout.razor

@@ -95,6 +95,12 @@
                         </div>
                         <span class="nav-text">Koncentry</span>
                     </NavLink>
+                    <NavLink class="nav-item" href="/creator" @onclick="CloseMobileMenu" title="Kreator" aria-label="Kreator">
+                        <div class="nav-icon">
+                            <NexusIcon Name="edit" Size="20" />
+                        </div>
+                        <span class="nav-text">Kreator</span>
+                    </NavLink>
                 </nav>
 
                 <div class="sidebar-footer">