refactor(AccountBackupStep1): migrate to design system components (#27580)

## **Description**

Migrate the `AccountBackupStep1` component from legacy styling patterns
to the design system:

- Replace `View` with `Box` and `StyleSheet.create()` with `twClassName`
Tailwind utilities
- Replace component-library `Button` and `Text` with
`@metamask/design-system-react-native` equivalents
- Remove redundant default props (`flexDirection Column`,
`justifyContent FlexStart`)
- Clean up unused imports (`PropTypes`, `fontStyles`, `scaling`,
`StorageWrapper`)

This is a pure refactor with no behavioral changes — functional logic,
navigation, and analytics are untouched.

## **Changelog**

CHANGELOG entry: null

## **Related issues**

N/A

## **Manual testing steps**

```gherkin
Feature: AccountBackupStep1 screen

  Scenario: View secure wallet prompt after wallet creation
    Given the user has just created a new wallet
    When the user reaches the "Secure your wallet" screen
    Then they see the SRP illustration, explanation text, and two buttons

  Scenario: Proceed to manual backup
    Given the user is on the "Secure your wallet" screen
    When the user taps "Start"
    Then they navigate to ManualBackupStep1

  Scenario: Skip backup for later
    Given the user has no funds and is on the "Secure your wallet" screen
    When the user taps "Remind me later"
    Then the skip confirmation modal appears

  Scenario: Theme switching
    Given the user is on the "Secure your wallet" screen
    When the device is in dark mode
    Then the dark SRP illustration is displayed
```

## **Screenshots/Recordings**

|           | Before | After |
|-----------|--------|-------|
| **Light** | <img
src="https://github.com/user-attachments/assets/5f5d1695-387b-4827-94bd-f6f915aa250e"
width="300"/> | <img width="300" height="2622" alt="Simulator Screenshot
- iPhone 17 - 2026-03-19 at 15 55 40"
src="https://github.com/user-attachments/assets/22d25352-4159-41fd-b4d6-9ec0fa515e2b"
/> |
| **Dark** | <img
src="https://github.com/user-attachments/assets/932503c1-fab4-4068-bb36-839a1c5c65cc"
width="300"/> | <img width="300" height="2622" alt="Simulator Screenshot
- iPhone 17 - 2026-03-19 at 15 55 48"
src="https://github.com/user-attachments/assets/0e1bbd50-19b0-433b-bdc5-dee42fe90f1f"
/> |






## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk refactor limited to the `AccountBackupStep1` UI layer, but it
could cause minor layout/spacing regressions due to the styling and
component swap.
> 
> **Overview**
> Migrates `AccountBackupStep1` from legacy `StyleSheet`/`View` and
component-library `Button`/`Text` to the MetaMask design system (`Box`,
`Button`, `Text`) with Tailwind (`useTailwind`) styling.
> 
> Keeps the existing navigation/metrics/back-handler flows, while
updating padding/spacing rules and refreshing Jest snapshots to match
the new rendered component tree and style output.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
2ba0f3d. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: tylerc-consensys