Add weekly background Kyoto CBF sync task (#352)

[Aniket-pd]

Description

This PR implements the background CBF sync flow discussed in #352.

In short, the app now registers a BGProcessingTask and schedules it to run roughly once a week (with network + external power requirements). When the task runs, it attempts to load the wallet and perform a Kyoto sync if the wallet exists and Kyoto is the selected backend.

I also added the required Info.plist entries for background processing and updated keychain accessibility to .afterFirstUnlock so background execution can still read wallet backup data while the device is locked.

Notes to the reviewers

I kept this intentionally narrow to the core pieces needed for background sync:

• task registration/scheduling,
• task execution path,
• plist configuration,
• keychain accessibility prerequisite.

I did not add product-level logging in this PR, since #351 tracks that separately.

Changelog notice

Add weekly background Kyoto CBF sync using BGProcessingTask, including required background task configuration and keychain access changes needed for locked-device execution.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I have formatted my code with swift-format per .swift-format file

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• UI changes tested on small, medium, and large devices to ensure layout consistency

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR