feat(creator): overhaul Creator flow, editor duplication, and staging setup (#83)

This pull request completely overhauls the Creator editor flow, resolves the editor duplication race condition, aligns layout/styling themes in light and dark mode, and adds Docker staging setups.

### Key Changes
1. **Creator Flow Polish**: Redesigned the editor canvas to prevent double scrolling by delegating overflow to the editor canvas layer, updated styles to a premium aesthetic.
2. **Race Condition Prevention**: Resolved Crepe editor duplication when loading or switching chapters by tracking state via shared window maps (`window.editorCache`, `window.editorStates`) and checking `_lastInitializedEditorId` synchronously in Blazor.
3. **Theme Synchronization**: Integrated explicit theme initialization (`ThemeService.InitializeAsync()`) and anchored CSS isolation selectors to correctly sync with Light (Soft Sepia) and Deep Dark theme preferences.
4. **Staging Automation**: Created staging docker configurations with `--nexus-only` flag to allow iterative development without resetting PG/Neo4j database containers.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #83
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.Application/DTOs/Media/MediaDtos.cs

@@ -16,3 +16,18 @@ public record ValidateChapterResponse(string SanitizedContent);
 /// Response DTO containing the uploaded media file URL.
 /// </summary>
 public record UploadResultDto(string Url);
+
+/// <summary>
+/// Represents a structured JSON backup envelope stored in LocalStorage.
+/// </summary>
+public class LocalBackupEnvelope
+{
+    public Guid ChapterId { get; set; }
+    public DateTime Timestamp { get; set; }
+    public string MarkdownContent { get; set; } = string.Empty;
+}
+
+/// <summary>
+/// Request DTO for chapter autosaving.
+/// </summary>
+public record AutosaveChapterRequest(string MarkdownContent);