A number of people here are really expecting this to be some ghost or hallucination, but really with the type of story this seems to be it's probably more that she's just in Haruki's imagination, something she's given herself to help her through the stages of grief. And I can imagine that's going to be what this is about, and it will end with her accepting Sakura's death.
"This here is for you to die again (Haruki coming to terms with Sakura's death and accepting it) and for me to live once again (Haruki moving on with the memory of Sakura in her heart. or something. Wording it is hard)."
I imagine it as though the grief has her in a rut. She's pacing the same circle, over and over. This Sakura, the Sakura that "came back to life" through the journal, is her pulling herself out of that rut, only in the end to understand that Sakura would not have wanted her to simply waste away after her death in a perpetual state of grievance and despair. I can already see the "Thank you, Sakura. I'll be fine from now on" at the end. But that doesn't mean I'm not going to enjoy the ride in the meantime.
It's going to be bittersweet, but I bet it'll be fulfilling.
Want to bring this comment back up since after reading chapter 2 this is exactly how I feel. Haru's doing all this, but imagining that Sakura is here with her, to give herself something to do besides waste away. She's remembering Sakura's warmth, but at the same time running the mental exercises of "how would a dead person feel coming back after time gone" like with the streaming service.
The store trip is an indulgence in pretend, the ghost that can't interact with anything, despite "cooking" for Haru, she doesn't need a toothbrush, because Haru knows she isn't there, looping back to Haru doing this all herself. It's part of the fantasy.