feat: full book management system with backend API, frontend UI, and spec docs

- Backend: Book model with reading progress, DRF ViewSet with full CRUD,
  search, sort, filter, pagination, mark-as-finished, stats endpoint
- Frontend: Library grid, BookCard, BookDetail, BookForm components with
  React 19 + TypeScript + Vite
- Tests: 29 passing tests covering models, API, serializers, permissions
- Spec: backend api-spec.md and frontend component-spec.md in docs/

Closes crisleo-hermes/cloud-reader#3

backend/books/admin.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from django.contrib import admin
+
+from books.models import Book
+
+
+@admin.register(Book)
+class BookAdmin(admin.ModelAdmin):
+    """Admin configuration for the Book model."""
+
+    list_display = ["title", "author", "genre", "reading_status", "reading_progress", "owner", "updated_at"]
+    list_filter = ["reading_status", "genre"]
+    search_fields = ["title", "author"]
+    readonly_fields = ["reading_progress", "created_at", "updated_at"]
+    raw_id_fields = ["owner"]