swap: make ChainSelector row fixed to top of list

[greg-schrammel]

Fixes APP-####

What changed (plus any additional context for devs)

should we add some animation/separator/fade/idk to the list clipping behind this header?

Screen recordings / screenshots

https://github.com/user-attachments/assets/bca523e0-4933-48c7-901e-63190eb44f8e

What to test

[linear[bot]]

APP-1514 Make ChainSelector row fixed to top of list

[brunobar79]

Launch in simulator or device for 17e95caeb8897a51a69976a1c2fc2c1336798dd1

[brunobar79]

Launch in simulator or device for b54aa94f0a72b8898071d3569231ecc3b967dd4a

[sentry-io[bot]]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ React ErrorBoundary TypeError: undefined is not a function createRainbowStore$argument_0(src/state/assets/... View Issue

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