feat(infra): Docker-compose configuration and environment-specific security guards for Beta deployment to Test environment (#56)

This pull request introduces the dedicated containerized infrastructure and configuration for deploying NexusReader's beta version in the Test environment.

### Summary of Changes

1. **Docker Infrastructure & Secrets**:
   - **`docker-compose.test.yml`**: Configured dedicated database and auxiliary services (PostgreSQL 17, Qdrant, Neo4j) on isolated, non-standard ports to ensure zero conflict with the existing server configurations.
   - **`.env.test.template`**: Provided an environment variable template showing required setups, including mandatory database passwords, API keys, and admin custom passwords.
   - **`.gitignore`**: Excluded local `.env` files to prevent accidental commits of production or staging secrets.

2. **Database Hardening**:
   - Configured Neo4j with basic authentication (`IDriver` instantiation uses basic auth when credentials are provided in configuration).
   - Configured PostgreSQL to use mandatory authentication.
   - Configured the admin seeder (`DbInitializer.cs`) to dynamically use `NEXUS_ADMIN_PASSWORD` from environment variables, falling back to a default password in local Development only.

3. **Feature-Flagged Restrictions**:
   - **`appsettings.Test.json`**: Implemented `Features:AllowRegistration` and `Features:AllowPasswordReset` flags set to `false`.
   - **Middleware Enforcement (`Program.cs`)**: Intercepts requests to `/identity/register` and `/identity/forgotPassword` (and their MVC/form variations) and rejects them with a `403 Forbidden` response in restricted environments.
   - **OAuth Provisioning Guard (`Program.cs`)**: Blocks new account provisioning via Google OAuth callback by checking the `Features:AllowRegistration` configuration, redirecting users to the login page with a descriptive error.
   - **UI Protection (`Login.razor`, `Register.razor`)**: Conditionally hides registration/password reset links and intercepts manual navigation attempts to `/account/register` by redirecting to login with a warning.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #56
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Services/ReaderStateService.cs

@@ -0,0 +1,41 @@
+using NexusReader.UI.Shared.Models;
+
+namespace NexusReader.UI.Shared.Services;
+
+/// <summary>
+/// Thread-safe implementation of IReaderStateService.
+/// Thread safety is ensured via lock-guarded property getters/setters.
+/// UI updates originating from the JS event loop (via JSInvokable) are synchronized at Blazor's InvokeAsync(StateHasChanged) render boundary.
+/// </summary>
+public sealed class ReaderStateService : IReaderStateService
+{
+    private readonly object _lock = new();
+    private int _scrollPercent;
+    private List<string> _checkpoints = new();
+    private string _blockId = string.Empty;
+    private MobileReaderTab _activeTab = MobileReaderTab.Reader;
+
+    public int CurrentScrollPercentage
+    {
+        get { lock (_lock) return _scrollPercent; }
+        set { lock (_lock) _scrollPercent = value; }
+    }
+
+    public List<string> CurrentCheckpoints
+    {
+        get { lock (_lock) return _checkpoints; }
+        set { lock (_lock) _checkpoints = value ?? new(); }
+    }
+
+    public string CurrentBlockId
+    {
+        get { lock (_lock) return _blockId; }
+        set { lock (_lock) _blockId = value ?? string.Empty; }
+    }
+
+    public MobileReaderTab ActiveTab
+    {
+        get { lock (_lock) return _activeTab; }
+        set { lock (_lock) _activeTab = value; }
+    }
+}