Add Autobuy button on the wallet/exchange screen

[thibistaken]

Add an Autobuy (recurring buy / DCA) entry-point button on the wallet / exchange screen so users can start an Autobuy directly from there, matching the web exchange. Today the recurring-buy flow is reachable but there's no prominent button on the wallet/exchange view — this is a feature-parity gap with the web app.

User story

As a user on the wallet / exchange screen, when I want to set up a recurring buy, I see an Autobuy button right there and can start the flow without hunting for it.

Acceptance criteria

• Autobuy button is present on the wallet / exchange screen
• Tapping it opens the existing recurring-buy (DCA) flow
• Placement and labeling are consistent with the web exchange and the app's other buy entry points
• Button respects existing KYC / funding gating (no dead-ends)

Backend API: Not required (reuses the existing recurring-buy flow).

[thibistaken]

Test cases

Happy path

• On the wallet / exchange screen, tap the Autobuy button → the recurring-buy (DCA) flow opens.

Placement / parity

• Button is visible and labeled consistently with the web exchange and other buy entry points.
• Existing entry points to the recurring-buy flow still work (no regression / no duplicate broken paths).

Gating / edge cases

• User not KYC'd or below required tier → button leads to the correct prompt, no dead-end.
• User with no funding → flow surfaces the funding prompt rather than erroring.
• Fresh account with no prior Autobuy → flow starts in create state.

Regression

• Wallet / exchange screen layout unaffected on small and large screens.
• Buy and Sell entry points unchanged.