[Cashtab Extension] Increase height to improve app->extension UX

Summary:
The recent additions of the cashtab message and opreturnraw toggles plus the output textarea have pushed the Send button to just beyond the existing dimensions, causing the users to scroll down to Send.

This diff increases the height to facilitate a smoother app->extension UX.

Current NOTIFICATION_HEIGHT @ 750

New NOTIFICATION_HEIGHT @ 950

Test Plan: Use an external app to trigger a BIP21 transaction to the extension and observe the Send button is in view without needing to scroll.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16047