feat: generate Zcash transparent addresses on demand

[Arbeitszeit]

Issue Number (if Applicable): N/A

Description

This is an experimental, stacked proposal that adds on-demand generation of disposable Zcash transparent addresses.

It currently depends on #3371. The branch includes that PR's metadata commit and should be rebased after #3371 is merged.

Design

Cake Wallet already derives disposable transparent addresses by creating indexed accounts from its offset-seed scheme. This proposal exposes creation of the next indexed account through the receive-page rotation button.

To keep seed recovery deterministic:

• Manual generation stops before 20 consecutive unused derived accounts exist.
• A restored wallet scans/derives accounts until it finds 20 consecutive unused addresses.
• The normal background pool continues to maintain at least five usable addresses.
• Recovery does not delete derived accounts.

The UI awaits generation, prevents duplicate button presses, selects the newly generated address, and displays generation or gap-limit errors in a snackbar.

Open review questions

• Is the existing offset-seed account derivation acceptable as a long-term public address-generation mechanism?
• Is a gap limit of 20 appropriate for Cake Wallet's Zcash recovery model?
• Should manual generation live behind an advanced setting rather than the receive-page rotation action?

Validation required

• Generate multiple transparent addresses on a physical device.
• Receive funds on non-consecutive generated addresses.
• Restore the seed on a clean installation and verify address/fund discovery.
• Verify automatic shielding and address rotation after recovery.
• Run TalkBack and compact-screen checks.

Only git diff --check was available locally. Native Zcash and device testing must be completed before this draft is considered merge-ready.

Pull Request - Checklist

• Initial Manual Tests Passed
• Double check modified code and verify it with the feature/task requirements
• Format code
• Look for code duplication
• Clear naming for variables and methods
• Manual tests in accessibility mode (TalkBack on Android) passed