feat(ui): implement premium gamified Concepts Map dashboard, unify design tokens, and enforce scoped CSS (#54)

Resolves #55

# gamified Concepts Map Dashboard, Architecture Cleanup, & CSS Unification

This Pull Request completes the gamified **Concepts Map** interactive experience and executes a strict visual and architectural clean-up of the **NexusReader SaaS** platform by consolidating styling around the centralized **Nexus Neon** design system, enforcing Native AOT-compliant scoped-CSS, and resolving style drift.

---

### 🚀 Key Implementations

#### 1. Gamified Interactive Concepts Map Dashboard
* **Dynamic Skill-Tree Visualizer:** Implemented a gamified node-based interactive tree that reads concept connections dynamically, rendering progress nodes, unlocked/locked states, and active visual connection lines.
* **Premium UX Details:** Integrated high-fidelity hover effects, detailed sidebar popovers showing unlocked stats/summaries, and smooth parallax backdrops.
* **Secure Multi-Tenant Gating:** Hardened data queries using explicit `TenantId` gating to ensure complete layout isolation between system tenants.

#### 2. Architecture & Service Optimization
* **Service Abstraction (`IConceptsMapService`):** Introduced a clean service interface decoupled from the UI layers.
* **Polyglot Fallback Implementations:**
  - **WasmConceptsMapService:** Implements efficient client-side fetching with error recovery/loading states.
  - **ServerConceptsMapService:** Handles deep database graph mapping, relationship resolution, and multi-tenant checks.
* **CQRS Integrity:** Enforced the CQRS Result Pattern by using `MediatR` handlers to fetch data structures instead of placing database queries in UI modules.

#### 3. Nexus Neon CSS Unification & Zero-Style-Tag Standards
* **Central Design Tokens:** Solidified typography (`Inter` / `Merriweather`), primary brand green hues (`var(--nexus-neon)` at `#00ff99`), and glow variables inside the global `app.css`.
* **Zero Inline Style Tags:** Standardized all dashboard modules by completely eliminating inline `<style>` blocks in favor of scoped `.razor.css` companion files.
* **Consolidated Buttons & Glass Panels:**
  - Standardized `.btn-nexus`, `.btn-nexus-primary`, and `.btn-nexus-secondary` throughout header/footer/card operations.
  - Removed duplicate `.glass-panel` background, blur, and support-declaration overrides, making components cleanly inherit standard global styles.
* **Eliminated Brand Splitting:** Resolved legacy purple-indigo user avatar and conversation bubble gradients inside the AI Intelligence Hub (`Intelligence.razor.css`), migrating them to premium glassmorphic surfaces (`rgba(255, 255, 255, 0.05)`) contrasting beautifully against the emerald green AI theme.

---

### 🧪 Verification & Build Gate Status

* **Successful Compilation Check:** Run `dotnet build NexusReader.slnx --no-restore` from the workspace root. Flawlessly succeeded with **zero compilation errors** (`Liczba błędów: 0`) under target `.NET 10`.
* **Verified Skills Synchrony:** The companion design guidelines skill (`nexus-ui-engine/SKILL.md`) has been fully updated to secure these layout and styling conventions for future dashboard features.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #54
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.Infrastructure/DependencyInjection.cs

@@ -121,6 +121,7 @@ public static class DependencyInjection
         // Fix #1: Ebook repository (scoped, matches AppDbContext lifetime)
         services.AddScoped<IEbookRepository, EbookRepository>();
         services.AddScoped<IQuizResultRepository, QuizResultRepository>();
+        services.AddScoped<IConceptsMapReadRepository, ConceptsMapReadRepository>();
 
         // Fix #2: SignalR broadcaster (scoped, wraps IHubContext which is itself a singleton wrapper)
         services.AddScoped<ISyncBroadcaster, SignalRSyncBroadcaster>();

src/NexusReader.Infrastructure/Persistence/ConceptsMapReadRepository.cs

@@ -0,0 +1,48 @@
+using Microsoft.EntityFrameworkCore;
+using NexusReader.Application.Abstractions.Persistence;
+using NexusReader.Data.Persistence;
+using NexusReader.Domain.Entities;
+
+namespace NexusReader.Infrastructure.Persistence;
+
+/// <summary>
+/// EF Core implementation of <see cref="IConceptsMapReadRepository"/>.
+/// Uses <see cref="IDbContextFactory{TContext}"/> for Blazor-safe scoped context creation.
+/// </summary>
+internal sealed class ConceptsMapReadRepository : IConceptsMapReadRepository
+{
+    private readonly IDbContextFactory<AppDbContext> _dbContextFactory;
+
+    public ConceptsMapReadRepository(IDbContextFactory<AppDbContext> dbContextFactory)
+    {
+        _dbContextFactory = dbContextFactory;
+    }
+
+    /// <inheritdoc />
+    public async Task<string?> GetLastReadPageIdAsync(string userId, CancellationToken cancellationToken = default)
+    {
+        await using var dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
+
+        var user = await dbContext.Users
+            .Where(u => u.Id == userId)
+            .Select(u => new { u.LastReadPageId })
+            .FirstOrDefaultAsync(cancellationToken);
+
+        return user?.LastReadPageId;
+    }
+
+    /// <inheritdoc />
+    public async Task<List<KnowledgeUnit>> GetKnowledgeUnitsForBookAsync(
+        Guid bookId,
+        string tenantId,
+        CancellationToken cancellationToken = default)
+    {
+        await using var dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
+
+        return await dbContext.KnowledgeUnits
+            .Where(k => k.EbookId == bookId &&
+                        (k.TenantId == tenantId || k.TenantId == "global" || string.IsNullOrEmpty(k.TenantId)))
+            .OrderBy(k => k.CreatedAt)
+            .ToListAsync(cancellationToken);
+    }
+}