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.Domain/Entities/Book.cs

@@ -0,0 +1,39 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace NexusReader.Domain.Entities;
+
+/// <summary>
+/// Represents a Book metadata entry that references its decoupled revisions.
+/// </summary>
+public class Book
+{
+    [Key]
+    public Guid Id { get; set; } = Guid.NewGuid();
+
+    [Required]
+    [MaxLength(255)]
+    public string Title { get; set; } = string.Empty;
+
+    [Required]
+    [MaxLength(128)]
+    public string TenantId { get; set; } = "global";
+
+    [Required]
+    public string UserId { get; set; } = string.Empty;
+
+    [ForeignKey(nameof(UserId))]
+    public virtual NexusUser? User { get; set; }
+
+    public Guid? CurrentDraftRevisionId { get; set; }
+
+    [ForeignKey(nameof(CurrentDraftRevisionId))]
+    public virtual BookRevision? CurrentDraftRevision { get; set; }
+
+    public Guid? LivePublishedRevisionId { get; set; }
+
+    [ForeignKey(nameof(LivePublishedRevisionId))]
+    public virtual BookRevision? LivePublishedRevision { get; set; }
+
+    public virtual ICollection<BookRevision> Revisions { get; set; } = new List<BookRevision>();
+}