style: complete Light Sepia theme overrides for user dashboard (#78)

Resolves #79

This Pull Request completes the visual implementation of the premium **"Warm Paper / Soft Sepia"** light theme across all user dashboard sub-modules and components.

### 🎨 Styling Refactoring & Light Sepia Layout
We refactored isolated component styles (`.razor.css`) to ensure proper contrast, remove hardcoded dark tokens, and fix text visibility under the `.theme-light` environment:

1. **Dashboard & Sidebar Layouts:**
   * **MainHubLayout.razor.css** & **Dashboard.razor.css**: Replaced hardcoded dark backgrounds with `var(--bg-surface)` and `var(--bg-base)`. Overrode user name brackets, progress bar elements, active quiz cards, graph nodes, and buttons.
2. **Catalog & Library Pages:**
   * **Catalog.razor.css** & **MyBooks.razor.css**: Adjusted cover hover actions, overlay transparency, and progress tracks. Fixed course tile background gradients to use a warm, elegant `#e4e1d9` layer and `var(--text-main)` code text.
3. **Profile & Settings Views:**
   * **Profile.razor.css** & **Settings.razor.css**: Overrode token usage progress tracks, page title gradient transparency, section descriptions, and diagnostic button styling.
4. **Concepts Dashboard & Interactive Widgets:**
   * **ConceptsDashboard.razor.css** & **ConceptsMap.razor.css**: Transitioned node headers, unlocked/locked badges, warning blocks, and term pills to semantic colors. Removed neon glow animations for locked nodes.
5. **Intelligence Workspace & AI Responses:**
   * **Intelligence.razor.css** & **AiResponseRenderer.razor.css**: Refactored empty state welcome messages, placeholder styles, input fields, and robot avatar borders. Refined the linear-gradient mask fade effect to blend correctly into the light sepia surface environment rather than dropping into dark transparent channels.
6. **Dashboard Sidebar Widgets:**
   * **CurrentReadingWidget.razor.css** & **ContextualRecommendationsWidget.razor.css**: Replaced hardcoded `#1a1a1e` card containers and light text colors with semantic variables (`var(--bg-surface)`, `var(--border)`, `var(--text-main)`, and `var(--text-muted)`).

### 🛠️ Blazor CSS Isolation Compiler Compliance
* Avoided the use of `:global(.theme-light)` selector overrides within isolated CSS rules because they are unsupported by Blazor's CSS isolation compiler and cause the compiled stylesheet to ignore them.
* Replaced them with the correct standard selector format `.theme-light .some-class` which properly compiles to `.theme-light .some-class[b-xxxx]`, applying correct theme styles recursively to child scoped markup.

### 🧪 Verification
* Checked that the solution builds cleanly with 0 compiler errors: `dotnet build NexusReader.slnx --no-restore`
* Ran all unit tests successfully: `dotnet test NexusReader.slnx --no-restore`

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #78
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

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

@@ -210,3 +210,60 @@
         align-items: center;
     }
 }
+
+/* ============================================================
+   LIGHT THEME OVERRIDES — "Warm Paper / Soft Sepia"
+   ============================================================ */
+
+.theme-light .current-reading-card {
+    background: var(--bg-surface);
+    border: 1px solid var(--border);
+}
+
+.theme-light .current-reading-card:hover {
+    background: var(--bg-surface);
+    border-color: var(--accent);
+    box-shadow: 0 10px 30px rgba(139, 130, 115, 0.12);
+}
+
+.theme-light .book-cover img {
+    box-shadow: 0 15px 35px rgba(139, 130, 115, 0.18);
+    border: 1px solid rgba(0, 0, 0, 0.06);
+}
+
+.theme-light .book-title {
+    color: var(--text-main);
+}
+
+.theme-light .author-name {
+    color: var(--text-muted);
+}
+
+.theme-light .chapter-name {
+    color: var(--text-main);
+}
+
+.theme-light .progress-bar-container {
+    background: #e4e1d9;
+}
+
+.theme-light .progress-bar-fill {
+    box-shadow: 0 0 6px rgba(16, 185, 129, 0.2);
+}
+
+.theme-light .book-excerpt {
+    color: var(--text-muted);
+}
+
+.theme-light .empty-text h3 {
+    color: var(--text-main);
+}
+
+.theme-light .empty-text p {
+    color: var(--text-muted);
+}
+
+.theme-light .empty-icon {
+    color: var(--accent);
+    filter: none;
+}