feat(dashspend): fixed denominations

[Syn-McJ]

Issue being fixed or feature implemented

We want to support fixed denomination merchants.

What was done?

• UI for fixed denominations
• Refactoring to allow updating merchant info before opening the Buy screen
• Network reachability and error handlers
• Align text with Android

How Has This Been Tested?

QA

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[Syn-McJ]

@CodeRabbit review

[HashEngineering]

LGTM