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/Services/IThemeService.cs

@@ -4,5 +4,6 @@ public interface IThemeService
 {
     bool IsLightMode { get; }
     event Func<Task>? OnThemeChanged;
+    Task InitializeAsync();
     Task ToggleTheme();
 }

src/NexusReader.UI.Shared/Services/ThemeService.cs

@@ -1,13 +1,42 @@
+using Microsoft.JSInterop;
+
 namespace NexusReader.UI.Shared.Services;
 
 public sealed class ThemeService : IThemeService
 {
+    private readonly IJSRuntime _jsRuntime;
     public bool IsLightMode { get; private set; } = false;
     public event Func<Task>? OnThemeChanged;
 
+    public ThemeService(IJSRuntime jsRuntime)
+    {
+        _jsRuntime = jsRuntime;
+    }
+
+    public async Task InitializeAsync()
+    {
+        try
+        {
+            IsLightMode = await _jsRuntime.InvokeAsync<bool>("themeInterop.isLightMode");
+            if (OnThemeChanged != null) await OnThemeChanged();
+        }
+        catch
+        {
+            // Fail silently during prerendering or if JS is not available yet
+        }
+    }
+
     public async Task ToggleTheme()
     {
         IsLightMode = !IsLightMode;
+        try
+        {
+            await _jsRuntime.InvokeVoidAsync("themeInterop.setLightMode", IsLightMode);
+        }
+        catch
+        {
+            // Fail silently
+        }
         if (OnThemeChanged != null) await OnThemeChanged();
     }
 }