feat(collections): add WebSocket broadcast on RemoveBook operation

Add real-time synchronization for collection book removal:
- Extract user ID from context for targeted broadcasts
- Broadcast 'collection_updated' message to user's other devices
- Includes collection_id, action, and book_id in message payload

This ensures that when a user removes a book from a collection,
all their connected devices (browser tabs, mobile apps, etc.)
receive real-time updates via WebSocket.

Consistent with existing AddBooks and BulkRemoveBooks operations
which already use BroadcastToUser for synchronization.

internal/handlers/collections.go

@@ -356,11 +356,28 @@ func (h *CollectionHandler) RemoveBook(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid book id"})
 	}
 
+	// Get user from context for WebSocket broadcast
+	user := c.Get("user").(database.Users)
+	userUUID := uuid.UUID(user.ID.Bytes)
+
 	err = h.collectionService.RemoveBookFromCollection(c.Request().Context(), collectionID, bookID)
 	if err != nil {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}
 
+	// Broadcast to user's other devices via WebSocket
+	if h.connManager != nil {
+		h.connManager.BroadcastToUser(userUUID.String(), wsync.BroadcastMessage{
+			Type:      "collection_updated",
+			Timestamp: time.Now().Format(time.RFC3339),
+			Data: map[string]interface{}{
+				"collection_id": collectionID.String(),
+				"action":        "book_removed",
+				"book_id":       bookID.String(),
+			},
+		})
+	}
+
 	return c.NoContent(http.StatusNoContent)
 }