Situation A (since it's kinda ambiguous who said what):
Shouko: I'm cold
Seri: I'll warm you up.
Seri (Because the last bubble is pointed to Seri): okay (??????)
Situation B:
Shouko: I'm cold
Shouko: I'll warm you up (??????)
Situation C:
Seri: I'm cold (she just got here while Shouko has been there for a while?)
What I think actually happened:
Shouko: I'm cold
Shouko again: warm me up
Seri: okay. squeeze
The 2nd line is also almost definitely said by Shouko, because I assume the "ah." in the background is Seri's
But idk Japanese so idk
Seri was outside. She can be cold too. In reality, she just wants an excuse to hug. She says "I'm cold," then Shouko says "I'll warm you up," then Seri says "Okay."
If Seri said both quotes they wouldn't give them separate speech bubbles right next to each other. Shouko also has the action swipe, she is the one doing the hugging, while Seri only hugs back after the fact.
Seri is craving affection and worried about Shouko's feelings. The scene works because Shouko is the one waiting for her, giving her the gift and doing the hugging. It's validation and a rejection of the worries Seri had in the chapter. Shouko is showing affection.