feat(creator): Refactor Creator flow, implement book creation pipeline & versioning, and setup Docker staging

- Relocate dashboard routing to /creator and editor workspace to /creator/edit/{BookId}
- Implement CreateBookCommand and handler with transactional default chapter seeding
- Implement PublishBookVersionCommand and GetCreatorDashboardDataQuery
- Build CreatorDashboard modal and UI components with customized dark input styles
- Add run-stage.sh script to automate staging environment setup, database migrations, and health checks
- Update developer workflow rules in GEMINI.md

src/NexusReader.Application/Features/Books/Commands/CreateBookCommand.cs

@@ -0,0 +1,18 @@
+using System;
+using NexusReader.Application.Abstractions.Messaging;
+
+namespace NexusReader.Application.Features.Books.Commands;
+
+/// <summary>
+/// Command to create a new Book, initialize its first Working Draft revision, and seed it with a default Introduction chapter.
+/// </summary>
+/// <param name="Title">The title of the new book.</param>
+/// <param name="Description">An optional description of the book.</param>
+/// <param name="UserId">The ID of the creator user.</param>
+/// <param name="TenantId">The tenant ID for multi-tenant isolation.</param>
+public record CreateBookCommand(
+    string Title,
+    string? Description,
+    string UserId,
+    string TenantId
+) : ICommand<Guid>;