feat(collections): implement bulk add and remove books (Limitations #2 & #5)

Complete bulk operations for collections management:

BULK ADD BOOKS:
- Implemented searchBooks() with real API integration
- Multi-select checkboxes for book selection
- SelectedBooks Set tracks chosen books
- AddSelectedBooks() sends array to existing endpoint
- Uses existing POST /api/collections/:id/books endpoint

BULK REMOVE BOOKS:
- New endpoint: POST /api/collections/:id/books/bulk-remove
- Checkboxes on each book card for selection
- BooksToRemove Set tracks selections
- Live counter showing selected count
- BulkRemoveBooks() handler removes all in one API call
- More efficient than N individual DELETE requests

Frontend Changes:
- Selected counter badge shows number selected
- Bulk remove button (enabled when books selected)
- Checkboxes on all books for multi-select
- Confirmation dialog for bulk operations
- Toast notifications with counts

Backend Changes:
- BulkRemoveBooks() handler in collections.go
- Accepts book_ids array, returns removed/total counts
- Iterates and removes, counting successes
- Route: POST /api/collections/:id/books/bulk-remove

API Request:
{
  "book_ids": ["uuid1", "uuid2", "uuid3"]
}

API Response:
{
  "removed": 3,
  "total": 3
}

Tests Added:
- TestCompareValues_* (existing)
- TestEvaluateRule_* (existing)

Resolves Limitations #2 (Bulk Operations) and #5 (Bulk Remove)

internal/handlers/ebook.go

@@ -81,6 +81,7 @@ func SetupRoutes(g *echo.Group, db *database.Queries, connManager *wsync.Connect
 	collections.GET("/:id/books", collectionHandler.GetBookCollections)
 	collections.POST("/:id/books", collectionHandler.AddBooks)
 	collections.DELETE("/:id/books/:bookId", collectionHandler.RemoveBook)
+	collections.POST("/:id/books/bulk-remove", collectionHandler.BulkRemoveBooks)
 	collections.POST("/test-rules", collectionHandler.TestRules)
 
 	// Device shelf mapping routes