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- Align perps activity items with perps activity UI, showing order
amount instead of PnL and increasing displayed decimals
- Update referral code text in onboarding flow

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CHANGELOG entry:

Fixes:

```gherkin
Feature: my feature name

  Scenario: user [verb for user action]
    Given [describe expected initial app state]

    When user [verb for user action]
    Then [describe expected outcome]
```

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- [ ] 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).
- [ ] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
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guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Perps activity details now display formatted asset amounts (up to 5
decimals) instead of PnL, with updated tests and referral onboarding
strings across locales.
>
> - **Rewards**:
> - **Perps event details**: `getPerpsEventDetails` now shows formatted
asset amounts (max 5 decimals) for `OPEN_POSITION`, `CLOSE_POSITION`,
`TAKE_PROFIT`, `STOP_LOSS` instead of PnL; includes direction for open.
> - **Number formatting**: `formatNumber` accepts optional `decimals`
and uses it for `maximumFractionDigits`.
> - **Tests**: Update `eventDetailsUtils.test.ts` expectations and test
names to reflect amount-based details and 5-decimal rounding (including
zero/low-amount cases).
> - **Localization**: Tweak referral onboarding copy; adjust/remove
`step4_title_referral_bonus` and `step4_referral_bonus_description` in
non-English locales; refine English strings.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
429f629. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Christian Montoya <christian.montoya@consensys.net>