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.Application/Abstractions/Services/IEpubReader.cs

@@ -20,4 +20,17 @@ public interface IEpubReader
         int chapterIndex,
         string? userId = null,
         CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Retrieves a resource (like an image) from the EPUB as a byte array.
+    /// </summary>
+    /// <param name="ebookId">The unique ID of the ebook to read.</param>
+    /// <param name="resourcePath">The path of the resource within the EPUB archive.</param>
+    /// <param name="userId">The authenticated user's ID (used for tenant isolation).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<Result<byte[]>> GetEpubResourceAsync(
+        Guid ebookId,
+        string resourcePath,
+        string? userId = null,
+        CancellationToken cancellationToken = default);
 }