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/Components/Atoms/NexusIcon.razor

@@ -10,6 +10,7 @@
             <line x1="8" y1="2" x2="8" y2="18" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             <line x1="16" y1="6" x2="16" y2="22" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             break;
+        case "share":
         case "share-2":
             <circle cx="18" cy="5" r="3" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             <circle cx="6" cy="12" r="3" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
@@ -45,6 +46,7 @@
             <line x1="3" y1="9" x2="21" y2="9" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             <line x1="9" y1="21" x2="9" y2="9" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             break;
+        case "book":
         case "book-open":
             <path d="M2 3h6a4 4 0 0 1 4 4v14a3 3 0 0 0-3-3H2z" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             <path d="M22 3h-6a4 4 0 0 0-4 4v14a3 3 0 0 1 3-3h7z" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
@@ -86,6 +88,17 @@
         case "log-out":
             <path d="M9 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h4M16 17l5-5-5-5M21 12H9" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
             break;
+        case "chevron-left":
+            <polyline points="15 18 9 12 15 6" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
+            break;
+        case "chevron-right":
+            <polyline points="9 18 15 12 9 6" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
+            break;
+        case "x":
+        case "close":
+            <line x1="18" y1="6" x2="6" y2="18" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
+            <line x1="6" y1="6" x2="18" y2="18" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" />
+            break;
         default:
             <!-- Fallback circle -->
             <circle cx="12" cy="12" r="10" />