Let's test before finishing the sewing. Use alligator clips to connect one leg of the photocell to D0 on GEMMA and the other to GND.
The pixels should animate pink when the sensor is exposed to light. Likewise they should turn off when the sensor is covered and completely dark.
To attach the LDR permanently, twist the ends of the leads into spirals as shown and paint the straight parts with nail polish-- it will insulate the LDR from the traces of conductive thread.
Stitch one spiral to the GND line around the inner perimeter of the heart (all GNDs are connected), and the other to D0 on GEMMA.
Toe knots and seal as before, then clip tails.