feat(ui/quiz): implement real-time global chapter quiz generation, submit results to database, and display dynamic statistics on dashboard (#53)

This PR fully implements the Global Chapter-Level Quiz Generation system in the NexusReader application.

### Key Accomplishments:
1. **SubmitQuizResultCommand**: Added MediatR command and handler to persist completed quiz results to the SQLite database securely, using our clean architecture result-pattern.
2. **Dynamic Dashboard Integration**: Re-engineered the user dashboard to fetch, calculate, and display real-time statistics (average score, total books read, total concept nodes mapped, and list of resolved quizzes with their dates and scores) directly from active database queries, eliminating static mockups.
3. **Haptic & Visual Feedback**: Enhanced the quiz flow with interactive CSS transitions, glowing hover feedback, and clear result visualization upon completion.
4. **Robust Verification**: Implemented comprehensive unit tests for `SubmitQuizResultCommandHandler` covering all success and failure/edge cases. Executed full `dotnet test` with 100% success rate.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #53
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Components/Organisms/BookIngestionModal.razor.css

@@ -377,6 +377,72 @@
     animation: spin 0.8s linear infinite;
 }
 
+/* Indexing State */
+.indexing-state {
+    flex: 1;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    border-radius: 12px;
+    background: rgba(255, 255, 255, 0.02);
+    border: 1px solid rgba(255, 255, 255, 0.05);
+    box-shadow: inset 0 0 12px rgba(255, 255, 255, 0.02);
+    position: relative;
+    overflow: hidden;
+    padding: 2rem;
+    animation: fadeIn 0.4s ease-out;
+}
+
+.indexing-content {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    text-align: center;
+    width: 100%;
+    gap: 1.25rem;
+}
+
+.indexing-content h3 {
+    margin: 0;
+    font-size: 1.25rem;
+    color: var(--nexus-neon, #00ffaa);
+    text-shadow: 0 0 10px rgba(0, 255, 153, 0.2);
+    letter-spacing: 0.5px;
+}
+
+.status-msg {
+    margin: 0;
+    font-size: 0.9rem;
+    color: var(--nexus-text-muted, #888);
+    min-height: 2.5rem;
+    line-height: 1.4;
+}
+
+.progress-bar-container {
+    width: 100%;
+    height: 8px;
+    background: rgba(255, 255, 255, 0.05);
+    border-radius: 4px;
+    overflow: hidden;
+    position: relative;
+    border: 1px solid rgba(255, 255, 255, 0.05);
+}
+
+.progress-bar-fill {
+    height: 100%;
+    background: linear-gradient(90deg, var(--nexus-neon, #00ffaa) 0%, #00b3ff 100%);
+    box-shadow: 0 0 10px rgba(0, 255, 153, 0.4);
+    border-radius: 4px;
+    transition: width 0.4s cubic-bezier(0.4, 0, 0.2, 1);
+}
+
+.percent {
+    font-family: var(--nexus-font-mono, monospace);
+    font-size: 1.1rem;
+    font-weight: 700;
+    color: var(--nexus-text);
+}
+
 @keyframes fadeIn {
     from { opacity: 0; }
     to { opacity: 1; }