fix: update Shield copy and error messages for consistency (#37869)

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## **Description**
Update Shield copy and error messages for consistency

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## **Related issues**

Fixes:

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## **Pre-merge author checklist**

- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
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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]
> Standardizes Shield-related localization by using “MetaMask
Transaction Shield,” lowercases the file uploader label, and refines
plan detail wording in both en and en_GB.
> 
> - **i18n (en, en_GB)**:
> - **Branding consistency**: Replace `Transaction Shield` with
`MetaMask Transaction Shield` across coverage alert messages (high risk,
chain not supported, paused, potential risks, signature not supported,
tx type not supported, unknown).
>   - **Copy tweaks**:
> - Update `shieldClaimFileUploader` label to `Image upload` (casing).
> - Revise `shieldTxDetails1Description` to “Secures assets on covered
transactions.”
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
d29d69b. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: hieu-w