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.Application/DTOs/Media/MediaDtos.cs

@@ -0,0 +1,18 @@
+namespace NexusReader.Application.DTOs.Media;
+
+// Note: These DTOs are registered in AppJsonContext.cs for JSON source generation.
+
+/// <summary>
+/// Request DTO for chapter validation/sanitization.
+/// </summary>
+public record ValidateChapterRequest(string Content);
+
+/// <summary>
+/// Response DTO containing sanitized chapter content.
+/// </summary>
+public record ValidateChapterResponse(string SanitizedContent);
+
+/// <summary>
+/// Response DTO containing the uploaded media file URL.
+/// </summary>
+public record UploadResultDto(string Url);