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.Web/Components/App.razor

@@ -9,6 +9,19 @@
     <link rel="stylesheet" href="NexusReader.Web.styles.css" />
     <ImportMap />
 
+    <script>
+        (function () {
+            const savedTheme = localStorage.getItem('theme');
+            const systemPrefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+            const isLight = savedTheme === 'light' || (!savedTheme && !systemPrefersDark);
+            if (isLight) {
+                document.documentElement.classList.add('theme-light');
+            } else {
+                document.documentElement.classList.remove('theme-light');
+            }
+        })();
+    </script>
+
     <HeadOutlet @rendermode="InteractiveAuto" />
 </head>
 
@@ -42,6 +55,7 @@
             setTimeout(hidePreloader, 3000);
         })();
     </script>
+    <script src="_content/NexusReader.UI.Shared/js/theme.js"></script>
     <script src="_content/NexusReader.UI.Shared/js/auth.js"></script>
 </body>