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.

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### 🚀 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.

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### 🧪 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.

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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.UI.Shared/Layout/AuthLayout.razor

@@ -4,21 +4,3 @@
     <link rel="stylesheet" href="_content/NexusReader.UI.Shared/css/nexus-auth.css" />
     @Body
 </div>
-
-<style>
-    .nexus-auth-shell {
-        position: fixed;
-        top: 0;
-        left: 0;
-        width: 100vw;
-        height: 100vh;
-        background-color: #121418 !important;
-        display: flex;
-        justify-content: center;
-        align-items: center;
-        z-index: 99999;
-        margin: 0;
-        padding: 0;
-        overflow: hidden;
-    }
-</style>

src/NexusReader.UI.Shared/Layout/AuthLayout.razor.css

@@ -0,0 +1,15 @@
+.nexus-auth-shell {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100vw;
+    height: 100vh;
+    background: radial-gradient(circle at center, #1b202e 0%, #0f1115 100%) !important;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    z-index: 99999;
+    margin: 0;
+    padding: 0;
+    overflow: hidden;
+}