<!--
Please submit this PR as a draft initially.
Do not mark it as "Ready for review" until the template has been
completely filled out, and PR status checks have passed at least once.
-->

## **Description**

<!--
Write a short description of the changes included in this pull request,
also include relevant motivation and context. Have in mind the following
questions:
1. What is the reason for the change?
2. What is the improvement/solution?
-->
Today, MetaMask supports a working buy deeplink that correctly routes
users across platforms. However, the equivalent sell deeplink is missing
on Extension, even though it exists on Mobile.
This PR Implements parity so that /sell deeplink works on Extension the
same way /buy does

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/40231?quickstart=1)

## **Changelog**

<!--
If this PR is not End-User-Facing and should not show up in the
CHANGELOG, you can choose to either:
1. Write `CHANGELOG entry: null`
2. Label with `no-changelog`

If this PR is End-User-Facing, please write a short User-Facing
description in the past tense like:
`CHANGELOG entry: Added a new tab for users to see their NFTs`
`CHANGELOG entry: Fixed a bug that was causing some NFTs to flicker`

(This helps the Release Engineer do their job more quickly and
accurately)
-->

CHANGELOG entry: Added a new sell deeplink

## **Related issues**

Fixes: https://consensyssoftware.atlassian.net/browse/GE-123

## **Manual testing steps**

1. Open
https://link.metamask.io/sell?chainId=59144&address=0x176211869cA2b568f2A7D4EE941E073a821EE1ff&amount=25
2. You should be redirected to the sell page, with USDC on Linea
selected. Amount is currently not correctly consumed by MMPortfolio =>
fix needed there

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

### **After**

<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/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
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/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**
> Adds a small, isolated route mapping that only constructs a redirect
URL and updates i18n strings, with minimal impact on core wallet or
security logic.
> 
> **Overview**
> Adds a new `/sell` deep link route, mirroring existing `/buy`
behavior, that redirects to the MetaMask Portfolio sell page and
forwards all query parameters.
> 
> Registers the new route in the deep-link route index and adds the
`deepLink_theSellPage` i18n string in `en` and `en_GB`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
173216c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->