fix: Update recipient placeholder text in send flow (#38236)

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## **Description**

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This PR aims to update placeholder text in send flow recipient
component.

## **Changelog**

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CHANGELOG entry: Update recipient placeholder text in send

## **Related issues**

Fixes: MetaMask/MetaMask-planning#6257

## **Manual testing steps**

1. Go to send pick any asset
2. See new placeholder text introduced in the component

## **Screenshots/Recordings**

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### **Before**

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### **After**

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## **Pre-merge author checklist**

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## **Pre-merge reviewer checklist**

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

> [!NOTE]
> Updates recipient input placeholder to support addresses or names,
with i18n key changes and e2e selector update.
> 
> - **Send flow UI**:
> - `recipient-input.tsx`: Switches placeholder to
`t('recipientPlaceholderText')` ("Enter or paste an address or name").
> - **i18n**:
>   - Remove `recipientPlaceholder` across locales.
> - Add `recipientPlaceholderText` in `app/_locales/en/messages.json`
and `en_GB` with updated copy.
> - **Tests**:
>   - `send-page.ts`: Update input selector to new placeholder text.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
10c0ddb. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: OGPoyraz