chore: switch to mobile optin endpoint (#20554)

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Migrates opt-in to a mobile timestamp-signed flow (EVM and Solana),
adds InvalidTimestamp retry, and updates service endpoints, types,
messenger, and tests.
> 
> - **Rewards opt-in flow**:
> - Replace challenge-based `optin` with timestamp-signed mobile flow in
`RewardsController` using `#signRewardsMessage` and
`RewardsDataService:mobileOptin`.
> - Add retry on `InvalidTimestampError` and support both EVM and Solana
signing paths.
> - **Data service**:
> - Add `mobileOptin(body)` calling `/auth/mobile-optin`; remove
`generateChallenge`/`optin` usage for controller path.
> - Return `InvalidTimestampError` with server timestamp; log error
payloads.
> - **Types**:
> - Remove `GenerateChallengeDto`, `ChallengeResponseDto`, `LoginDto`;
add `MobileOptinDto` and refine `MobileLoginDto`.
> - **Messenger**:
> - Replace allowed actions `generateChallenge`/`optin` with
`mobileOptin`.
> - **Tests**:
> - Update controller and service tests for mobile opt-in (EVM/Solana),
timestamp retry, and token storage handling.
>   - Minor fix: cast `base58.decode` Buffer to `Uint8Array` in tests.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
5b9aa5c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Rik Van Gulck <[email protected]>

Author: sophieqgu