Refactor `plot_2d_latent_walk`

Might be worth decoupling these two panels by just having the `plot_2d_latent_walk` method recalculate the PC1/2 latent walks, rather than relying on panel B to have calculated it

_Originally posted by @jessicasyu in https://github.com/AllenCell/endothelial-pipeline/pull/1821#discussion_r3691591837_

Inspect the implementation and callers of plot_2d_latent_walk, focusing on how panel B currently provides the PC1/2 latent walks. Confirm how the panels are assembled, then verify that plot_2d_latent_walk recalculates its own walks and that both panels still render correctly.

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