fix: preserve and render EPUB images via dynamic server endpoint (#65)

Fixes #64

### Summary of Changes
1. **Extended `IEpubReader` & `EpubReaderService`**: Added `GetEpubResourceAsync` to handle binary data extraction of static assets (like images) from the EPUB archive.
2. **Added Client-Side HTTP Call**: Extended `WasmEpubService` to retrieve static resources from the server using the API client.
3. **Preserved and Sanitized Images**: Updated `ExtractParagraphs` and `SanitizeParagraph` to treat `<img>` tags as first-class citizens, preserving their `src` attributes and excluding them from sanitization stripping.
4. **Dynamic URL Rewriting**: Introduced a relative-to-absolute path resolution algorithm (`ResolveRelativePath`) and rewrote image `src` attributes to use the dynamic endpoint `/api/epub/{ebookId}/resource?path=...`.
5. **Registered API Resource Serving Endpoint**: Added the `/api/epub/{ebookId:guid}/resource` minimal API endpoint in `Program.cs` that maps requests directly to `GetEpubResourceAsync` and returns files with the correct MIME type.
6. **Added Unit Tests**: Created `EpubReaderServiceTests.cs` to verify all image extraction, path resolution, and sanitization/rewriting rules. All tests pass successfully.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #65
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

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

@@ -0,0 +1,11 @@
+namespace NexusReader.UI.Shared.Services;
+
+/// <summary>
+/// Strongly-typed feature settings for the client UI layer.
+/// Used to decouple the UI from raw IConfiguration to prevent exposure of sensitive settings.
+/// </summary>
+public class FeatureSettings
+{
+    public bool AllowRegistration { get; set; } = true;
+    public bool AllowPasswordReset { get; set; } = true;
+}