feat(mobile-ux): implement theme toggle, client-side persistence, and light mode style overrides (#71)

Resolves #72

## Description

This PR implements the theme toggle mechanism, client-side persistence, and comprehensive light mode style overrides for the mobile reader layout in **NexusReader**.

### Key Changes
1. **ThemeService & State Management**:
   - Enhanced `ThemeService` with `InitializeAsync` and `ToggleTheme` using JS Interop.
   - Synchronized theme state changes via `OnThemeChanged` event.
2. **Local Storage Persistence & FOUC Prevention**:
   - Added client-side script in `App.razor` and MAUI `index.html` to instantly apply `.theme-light` from `localStorage` before prerendering/rendering to prevent a Flash of Unthemed Content (FOUC).
   - Created `theme.js` containing JS helper methods (`themeInterop.isLightMode`, `themeInterop.setLightMode`) to interface with `localStorage` and `document.documentElement` class list.
3. **UI Theme Toggle**:
   - Added a minimalist glassmorphic theme toggle button in the header of the `ReaderCanvas.razor` with dynamic transitions and icon morphing between sun and moon SVG icons.
4. **Light Mode Stylesheets Overrides**:
   - Added detailed light mode scoped styles (`.theme-light`) for the `ReaderCanvas` and the bottom-sheet `GlobalIntelligence` AI chat panel, utilizing earthy gray text (`#2d2a26`) on warm paper-like backgrounds (`rgba(244, 241, 234, 0.95)` / `#faf8f5`) to maintain high-quality aesthetic consistency.

### Verification
- Solution built successfully without errors (`dotnet build NexusReader.slnx --no-restore`).
- Scoped CSS isolation checked and validated for both Web and MAUI contexts.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #71
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

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

@@ -340,7 +340,7 @@
 @media (max-width: 768px) {
     .reader-canvas {
         padding-top: 54px !important;
-        padding-bottom: 80px !important;
+        padding-bottom: 120px !important;
         /* Ensure content is clear of bottom toolbar */
     }
 
@@ -350,6 +350,22 @@
     }
 }
 
+@media (max-width: 767px) {
+    ::deep .nexus-ebook,
+    ::deep .nexus-ebook p {
+        font-size: 16px !important;
+        line-height: 1.55 !important;
+    }
+
+    ::deep .nexus-ebook h1 {
+        font-size: 1.35rem !important;
+        line-height: 1.4 !important;
+        margin-top: 1.5rem !important;
+        margin-bottom: 1rem !important;
+    }
+}
+
+
 .nexus-mobile-reader-header {
     position: fixed;
     top: 0;
@@ -400,6 +416,7 @@
     gap: 0.25rem;
     height: 100%;
     min-width: 0;
+    margin-right: 40px; /* Space to prevent overlap with absolute theme toggle button */
 }
 
 .nexus-mobile-chapter-title {
@@ -463,4 +480,91 @@
 
 .theme-light .nexus-chapter-nav-btn:hover:not(:disabled) {
     background: rgba(0, 0, 0, 0.06);
+}
+
+/* Minimalist Theme Toggle Button with Glassmorphism */
+.nexus-theme-toggle-btn {
+    position: absolute;
+    right: 1rem;
+    top: 50%;
+    transform: translateY(-50%);
+    width: 36px;
+    height: 36px;
+    border-radius: 50%;
+    border: 1px solid rgba(255, 255, 255, 0.08);
+    background: rgba(255, 255, 255, 0.03);
+    backdrop-filter: blur(10px);
+    -webkit-backdrop-filter: blur(10px);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    cursor: pointer;
+    z-index: 1001;
+    padding: 0;
+    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+}
+
+.nexus-theme-toggle-btn:hover {
+    background: rgba(255, 255, 255, 0.08);
+    border-color: rgba(255, 255, 255, 0.15);
+    transform: translateY(-50%) scale(1.05);
+}
+
+.nexus-theme-toggle-btn:active {
+    transform: translateY(-50%) scale(0.95);
+}
+
+/* Theme specifics for Light Mode */
+.theme-light .nexus-theme-toggle-btn {
+    border-color: rgba(0, 0, 0, 0.08);
+    background: rgba(0, 0, 0, 0.03);
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.05);
+}
+
+.theme-light .nexus-theme-toggle-btn:hover {
+    background: rgba(0, 0, 0, 0.06);
+    border-color: rgba(0, 0, 0, 0.12);
+}
+
+/* Icon Styling and Transition */
+.theme-toggle-icon {
+    width: 18px;
+    height: 18px;
+    stroke-width: 2px;
+}
+
+/* In Dark Mode, the icon should be brand green (#10b981) */
+.theme-toggle-icon.moon {
+    color: #10b981;
+    filter: drop-shadow(0 0 4px rgba(16, 185, 129, 0.3));
+    animation: morphMoon 0.4s cubic-bezier(0.4, 0, 0.2, 1) forwards;
+}
+
+/* In Light Mode, the icon should be a dark earthy gray (#2d2a26) */
+.theme-toggle-icon.sun {
+    color: #2d2a26;
+    animation: morphSun 0.4s cubic-bezier(0.4, 0, 0.2, 1) forwards;
+}
+
+@keyframes morphMoon {
+    0% {
+        transform: rotate(-45deg) scale(0.7);
+        opacity: 0;
+    }
+    100% {
+        transform: rotate(0deg) scale(1);
+        opacity: 1;
+    }
+}
+
+@keyframes morphSun {
+    0% {
+        transform: rotate(45deg) scale(0.7);
+        opacity: 0;
+    }
+    100% {
+        transform: rotate(0deg) scale(1);
+        opacity: 1;
+    }
 }