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/Components/Atoms/NexusIcon.razor

@@ -101,6 +101,11 @@
                     <line x1="5" y1="12" x2="19" y2="12" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
                     <polyline points="12 5 19 12 12 19" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             break;
+        case "edit":
+        case "edit-2":
+                    <path d="M11 4H4a2 2 0 0 0-2 2v14a2 2 0 0 0 2 2h14a2 2 0 0 0 2-2v-7" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
+                    <path d="M18.5 2.5a2.121 2.121 0 1 1 3 3L12 15l-4 1 1-4 9.5-9.5z" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
+            break;
         case "log-out":
                     <path d="M9 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h4M16 17l5-5-5-5M21 12H9" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             break;