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/Pages/MyBooks.razor.css

@@ -19,13 +19,13 @@
     font-size: 2.5rem;
     font-weight: 700;
     margin: 0 0 0.5rem 0;
-    color: #ffffff;
+    color: var(--text-main);
     letter-spacing: -0.5px;
 }
 
 .header-title-section .subtitle {
     font-size: 1rem;
-    color: #a1a1aa;
+    color: var(--text-muted);
     margin: 0;
 }
 
@@ -67,27 +67,27 @@
     height: 100%;
     overflow: hidden;
     border-radius: 12px;
-    background: #1a1a1e;
-    border: 1px solid rgba(255, 255, 255, 0.05);
+    background: var(--bg-surface);
+    border: 1px solid var(--border);
     transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
     position: relative;
 }
 
 .book-card:hover {
     transform: translateY(-4px);
-    box-shadow: 0 12px 30px rgba(0, 0, 0, 0.3);
-    border-color: rgba(16, 185, 129, 0.2);
+    box-shadow: 0 12px 30px rgba(0, 0, 0, 0.1);
+    border-color: var(--accent);
 }
 
 .book-cover-container {
     position: relative;
     height: 360px;
-    background: rgba(0, 0, 0, 0.2);
+    background: rgba(0, 0, 0, 0.05);
     overflow: hidden;
     display: flex;
     align-items: center;
     justify-content: center;
-    border-bottom: 1px solid rgba(255, 255, 255, 0.05);
+    border-bottom: 1px solid var(--border);
 }
 
 .book-cover {
@@ -145,7 +145,7 @@
     font-size: 1.2rem;
     font-weight: 600;
     margin: 0 0 0.4rem 0;
-    color: #ffffff;
+    color: var(--text-main);
     overflow: hidden;
     text-overflow: ellipsis;
     white-space: nowrap;
@@ -154,7 +154,7 @@
 
 .book-author {
     font-size: 0.9rem;
-    color: #a1a1aa;
+    color: var(--text-muted);
     margin: 0 0 1.25rem 0;
 }
 
@@ -179,7 +179,7 @@
 
 .progress-bar {
     height: 6px;
-    background: rgba(255, 255, 255, 0.05);
+    background: var(--border);
     border-radius: 3px;
     overflow: hidden;
 }
@@ -192,7 +192,7 @@
 
 .progress-text {
     font-size: 0.8rem;
-    color: #a1a1aa;
+    color: var(--text-muted);
     white-space: nowrap;
     overflow: hidden;
     text-overflow: ellipsis;
@@ -232,14 +232,14 @@
     justify-content: center;
     padding: 5rem 2rem;
     text-align: center;
-    background: #1a1a1e;
-    border: 1px solid rgba(255, 255, 255, 0.05);
+    background: var(--bg-surface);
+    border: 1px solid var(--border);
     border-radius: 12px;
 }
 
 .empty-icon-pulse {
     margin-bottom: 2rem;
-    color: #a1a1aa;
+    color: var(--text-muted);
     animation: pulse 3s infinite alternate;
 }
 
@@ -247,11 +247,11 @@
     font-family: var(--nexus-font-serif);
     font-size: 1.8rem;
     margin: 0 0 0.5rem 0;
-    color: #ffffff;
+    color: var(--text-main);
 }
 
 .empty-state-container p {
-    color: #a1a1aa;
+    color: var(--text-muted);
     max-width: 400px;
     margin: 0 0 2rem 0;
 }
@@ -278,14 +278,14 @@
     border-radius: 12px;
     overflow: hidden;
     height: 480px;
-    background: #1a1a1e;
-    border: 1px solid rgba(255, 255, 255, 0.05);
+    background: var(--bg-surface);
+    border: 1px solid var(--border);
     opacity: 0.6;
 }
 
 .skeleton-cover {
     height: 360px;
-    background: linear-gradient(90deg, rgba(255,255,255,0.02) 25%, rgba(255,255,255,0.06) 50%, rgba(255,255,255,0.02) 75%);
+    background: linear-gradient(90deg, var(--bg-base) 25%, var(--border) 50%, var(--bg-base) 75%);
     background-size: 200% 100%;
     animation: loading 1.5s infinite;
 }
@@ -298,7 +298,7 @@
 }
 
 .skeleton-line {
-    background: linear-gradient(90deg, rgba(255,255,255,0.02) 25%, rgba(255,255,255,0.06) 50%, rgba(255,255,255,0.02) 75%);
+    background: linear-gradient(90deg, var(--bg-base) 25%, var(--border) 50%, var(--bg-base) 75%);
     background-size: 200% 100%;
     animation: loading 1.5s infinite;
     border-radius: 4px;
@@ -336,9 +336,9 @@
     gap: 1.25rem;
     padding: 1.25rem 2.25rem;
     border-radius: 40px;
-    background: rgba(13, 13, 15, 0.85);
+    background: var(--bg-surface);
     backdrop-filter: blur(16px);
-    border: 1px solid rgba(255, 255, 255, 0.08);
+    border: 1px solid var(--border);
     animation: scaleIn 0.3s cubic-bezier(0.34, 1.56, 0.64, 1);
 }
 
@@ -354,7 +354,7 @@
 
 .loader-text {
     font-weight: 500;
-    color: #ffffff;
+    color: var(--text-main);
     font-size: 0.95rem;
 }
 
@@ -383,3 +383,27 @@
     from { transform: translate(-50%, -50%) scale(0.9); opacity: 0; }
     to { transform: translate(-50%, -50%) scale(1); opacity: 1; }
 }
+
+/* ============================================
+   LIGHT THEME OVERRIDES — Warm Paper / Soft Sepia
+   ============================================ */
+
+.theme-light .book-card:hover {
+    box-shadow: 0 12px 30px rgba(139, 130, 115, 0.15);
+}
+
+.theme-light .book-cover-container {
+    background: rgba(0, 0, 0, 0.03);
+}
+
+.theme-light .cover-overlay {
+    background: rgba(0, 0, 0, 0.5);
+}
+
+.theme-light .book-card:hover .read-action {
+    color: #292524;
+}
+
+.theme-light .progress-bar {
+    background: #e4e1d9;
+}