Remove active Coinbase and Uphold DashSync dependencies

[llbartekll]

Summary

• add an app-owned KeychainStore based on the existing PinStore implementation while preserving DashSync-compatible service, account names, accessibility, and byte encoding
• rewire Coinbase, Uphold, DWGlobalOptions, and ExchangeAddressProvider keychain access
• move all reachable Uphold HTTP operations to the app-owned HTTPClient behind the existing Objective-C facade
• remove the unused Uphold provider, account model, and dead endpoint chains
• update migration documentation and keychain golden fixtures

Verification

• scripts/auth_keychain_goldens/run_goldens.sh
• plutil -lint DashWallet.xcodeproj/project.pbxproj
• dashpay Debug arm64 iOS Simulator build
• dashpay Release device archive with code signing disabled
• static audit for removed DashSync keychain/HTTP references and dead endpoint callers
• git diff --check

The dashwallet Debug/Release builds remain blocked by the pre-existing Watch runtime mismatch (23S303 vs SDK 23T570); the same failure is present on the clean base.

Manual smoke test

• confirm existing Coinbase and Uphold sessions survive upgrade and restart
• verify Uphold cards and balance
• buy/withdraw DASH, including OTP retry and commit
• verify logout/revoke and re-login
• cancel during re-quote without showing a new error UI

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/dashsync-coinbase-uphold-tail

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