fix: plan content view losing scroll position on unmount

[CaioDGallo]

Problem

• The plan content view in the conversation loses its scroll position when unmounted (e.g. scrolling until it leaves the screen).
• If there is a big plan, it can be annoying for the user to keep coming back to the plan and having to scroll back to where they were previously reading.

Changes

• Added a JavaScript Map for storing previous scroll positions of any PlanContent.tsx component present in the current conversation.
• Scroll now keeps its position even when multiple Plan iterations exist in the conversation history.

Closes #1298

[jonathanlab]

LGTM, thank you!

small nit: This also occurs with any other scrollable elements in the chat, such as diffs, but we can fix that in another PR.