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.Application/Commands/User/UpdateThemeCommandHandler.cs

@@ -0,0 +1,41 @@
+using FluentResults;
+using Microsoft.EntityFrameworkCore;
+using NexusReader.Application.Abstractions.Messaging;
+using NexusReader.Data.Persistence;
+
+namespace NexusReader.Application.Commands.User;
+
+public class UpdateThemeCommandHandler : ICommandHandler<UpdateThemeCommand>
+{
+    private readonly IDbContextFactory<AppDbContext> _dbContextFactory;
+
+    public UpdateThemeCommandHandler(IDbContextFactory<AppDbContext> dbContextFactory)
+    {
+        _dbContextFactory = dbContextFactory;
+    }
+
+    public async Task<Result> Handle(UpdateThemeCommand request, CancellationToken cancellationToken)
+    {
+        try
+        {
+            using var dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
+            var user = await dbContext.Users
+                .AsTracking()
+                .FirstOrDefaultAsync(u => u.Id == request.UserId, cancellationToken);
+                
+            if (user == null)
+            {
+                return Result.Fail("User not found.");
+            }
+
+            user.ThemePreference = request.Mode;
+            await dbContext.SaveChangesAsync(cancellationToken);
+            
+            return Result.Ok();
+        }
+        catch (Exception ex)
+        {
+            return Result.Fail(new Error("Failed to save theme preference in database.").CausedBy(ex));
+        }
+    }
+}