Promote the Friends browser extension

[akirk]

Summary

The Browser Extension settings page was only hot-linked from the extension itself, so there was no way for users to discover the extension exists from inside WordPress. This PR adds discovery surfaces:

• Friends menu — the Browser Extension submenu is now always visible (was previously registered only when the URL already targeted it).
• Browser Extension settings page — leads with a description of what the extension does plus Install buttons for Chrome and Firefox, then the API key below. Now works for both audiences (people exploring before install, people landing from the extension after install).
• Tools → Bookmarklets card — adds the Chrome link alongside Firefox and expands the one-line description.
• README.md — feature bullet under "You can…" plus a new FAQ entry, so the extension shows up on wordpress.org/plugins/friends.
• News page — new 4.1 entry covering the Twitter theme (shipped in 4.0.6) and the browser extension.

Test plan

• Visit Friends → Browser Extension and confirm the promo block, install buttons, and API key all render correctly.
• Confirm the Browser Extension item appears in the Friends submenu without needing to be on the page already.
• Visit Tools and confirm the bookmarklets card shows both Chrome and Firefox links.
• Visit Friends → Home (News) and confirm the new 4.1 entry appears at the top of the TOC with the Twitter theme and Browser Extension cards.
• Verify the install links open the correct Chrome Web Store and addons.mozilla.org pages.

Changelog

• Automatically create a changelog entry from the details below

Type

• Changed

Message

Surface the Friends browser extension in more places so users can discover it: the Browser Extension submenu is now always visible under Friends, the settings page describes what the extension does and links to the Chrome and Firefox stores, and the Tools → Bookmarklets card and README mention it as well.

[github-actions]

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This will install and activate the plugin with the changes from this PR.