Fix Launch Screen Background being in LightMode in DarkMode

[MarlowBrown]

First time contributor checklist

• I have read the README and CONTRIBUTING documents
• I have signed the Contributor Licence Agreement

Contributor checklist

• My commits are rebased on the latest main branch
• My commits are in nice logical chunks
• My contribution is fully baked and is ready to be merged as is
• I have tested my contribution on these devices:

• iPhone 16, iOS 18.2

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Description

Changed the background color of the Launch Screen to systemBackground color because Signal/BackgroundLevels/backgroundbase was missing. This fixes #5937.

I tested my fix by running the application in light and dark modes to ensure the correct colors for each respective mode.

[sashaweiss-signal]

Hey @MarlowBrown, thanks for the contribution and for signing the CLA. The way we merge public PRs is by re-opening the PR in our private repo where we do our primary development, making sure the commit includes your name and email for attribution purposes. Would you mind sharing the name/email you'd like associated with the commit I create in our private repo? If you'd prefer, feel free to send those to me directly at sasha@signal.org.

[sashaweiss-signal]

(I saw you had a number of PRs open – I'll plan to use the same name/email for all of them that we decide to merge!)

[MarlowBrown]

Hey @sashaweiss-signal, I send you an email with my attribution information.

[sashaweiss-signal]

This was merged internally as 17afb854c3, which should become public in a week or two with an upcoming release!