fix: add role img to app header picker network (#23842)

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## **Description**

To improve the accessibility of our ~Avatar components~, especially when
a screen reader is used, we propose adding the role="img" attribute to
these components. This enhancement aims to prevent screen readers from
reading out the fallback letter, providing a more meaningful and
contextually appropriate description. For instance, when tabbing through
networks in the extension, the screen reader should announce "S,
Sepolia" instead of just "S". This change will ensure our UI is more
accessible and user-friendly for individuals relying on screen readers.

Upon further investigation with @georgewrmarshall, we came to the
conclusion that it is best suited for the issue to be applied to the
`app header network picker`.

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## **Related issues**

Fixes: #23251 

## **Manual testing steps**

1. Turn on your computers screen reader (mac: CMD + F5)
2. Run local build of extension 
3. Tab to a component that uses an avatar component (e.g. see screenshot
for example component)

## **Screenshots/Recordings**

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

### **Before**


https://github.com/MetaMask/metamask-extension/assets/8112138/ec6ab28f-8fbf-4cd2-b0ea-787f82d7bcae

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

### **After**



https://github.com/MetaMask/metamask-extension/assets/26469696/bef063ff-41d7-4738-98b3-625013861d75




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

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## **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.

ui/components/multichain/app-header/__snapshots__/app-header.test.js.snap

@@ -207,12 +207,14 @@ exports[`App Header should match snapshot 1`] = `
     >
       <div>
         <button
+          aria-label="Network Menu Chain 5"
           class="mm-box mm-picker-network multichain-app-header__contents__network-picker mm-box--margin-2 mm-box--padding-right-4 mm-box--padding-left-2 mm-box--display-none mm-box--sm:display-flex mm-box--gap-2 mm-box--align-items-center mm-box--background-color-background-alternative mm-box--rounded-pill"
           data-testid="network-display"
           disabled=""
         >
           <div
             class="mm-box mm-text mm-avatar-base mm-avatar-base--size-xs mm-avatar-network mm-picker-network__avatar-network mm-text--body-xs mm-text--text-transform-uppercase mm-box--display-flex mm-box--justify-content-center mm-box--align-items-center mm-box--color-text-default mm-box--background-color-background-alternative mm-box--rounded-full mm-box--border-color-transparent box--border-style-solid box--border-width-1"
+            role="img"
           >
             C
           </div>

ui/components/multichain/app-header/app-header.js

@@ -237,13 +237,16 @@ export const AppHeader = ({ location }) => {
                     <PickerNetwork
                       avatarNetworkProps={{
                         backgroundColor: testNetworkBackgroundColor,
+                        role: 'img',
                       }}
                       className="multichain-app-header__contents--avatar-network"
                       ref={menuRef}
                       as="button"
                       src={currentNetwork?.rpcPrefs?.imageUrl}
                       label={currentNetwork?.nickname}
-                      aria-label={t('networkMenu')}
+                      aria-label={`${t('networkMenu')} ${
+                        currentNetwork?.nickname
+                      }`}
                       labelProps={{
                         display: Display.None,
                       }}
@@ -262,8 +265,12 @@ export const AppHeader = ({ location }) => {
                   <PickerNetwork
                     avatarNetworkProps={{
                       backgroundColor: testNetworkBackgroundColor,
+                      role: 'img',
                     }}
                     margin={2}
+                    aria-label={`${t('networkMenu')} ${
+                      currentNetwork?.nickname
+                    }`}
                     label={currentNetwork?.nickname}
                     src={currentNetwork?.rpcPrefs?.imageUrl}
                     onClick={(e) => {
@@ -482,7 +489,9 @@ export const AppHeader = ({ location }) => {
                 <PickerNetwork
                   avatarNetworkProps={{
                     backgroundColor: testNetworkBackgroundColor,
+                    role: 'img',
                   }}
+                  aria-label={`${t('networkMenu')} ${currentNetwork?.nickname}`}
                   label={currentNetwork?.nickname}
                   src={currentNetwork?.rpcPrefs?.imageUrl}
                   onClick={(e) => {