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/collections.go

@@ -306,6 +306,43 @@ func (h *CollectionHandler) RemoveBook(c echo.Context) error {
 	return c.NoContent(http.StatusNoContent)
 }
 
+type BulkRemoveBooksRequest struct {
+	BookIDs []string `json:"book_ids" validate:"required"`
+}
+
+func (h *CollectionHandler) BulkRemoveBooks(c echo.Context) error {
+	collectionID, err := uuid.Parse(c.Param("id"))
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid collection id"})
+	}
+
+	var req BulkRemoveBooksRequest
+	if err := c.Bind(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
+	}
+	if err := c.Validate(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
+	}
+
+	removedCount := 0
+	for _, bookIDStr := range req.BookIDs {
+		bookID, err := uuid.Parse(bookIDStr)
+		if err != nil {
+			continue
+		}
+
+		err = h.collectionService.RemoveBookFromCollection(c.Request().Context(), collectionID, bookID)
+		if err == nil {
+			removedCount++
+		}
+	}
+
+	return c.JSON(http.StatusOK, map[string]interface{}{
+		"removed": removedCount,
+		"total":   len(req.BookIDs),
+	})
+}
+
 func (h *CollectionHandler) GetDeviceMappings(c echo.Context) error {
 	deviceID, err := uuid.Parse(c.Param("id"))
 	if err != nil {