feat: Add OpenSearch support for browser integration

[kunalagra]

This PR adds support for the OpenSearch standard, allowing users to add Unduck as a search engine in their browser. The opensearch.xml is based on YouTube's one.

Changes:

• Adds an opensearch.xml file to the /public directory.
• Links to the XML file from index.html using .

To Test:

• Open this deployment - https://unduck-ten.vercel.app/
• Your browser should now offer to add "Unduck" as a search engine (e.g., via the address bar menu in Firefox or in Chrome's search engine settings).
• Add it and test a search from the address bar.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
unduck	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 8, 2025 3:48pm

[graphite-app]

The XML file is missing a trailing newline at the end. Adding a newline after the closing </OpenSearchDescription> tag would follow standard file formatting conventions and prevent potential issues with certain tools and version control systems. This is considered a best practice for text files in Unix/Linux environments.

Spotted by Diamond

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[h14h]

@t3dotgg Sorry for the @ but I very much want this PR to merge.

Would love to be able to use unduck.link in Vanadium on GrapheneOS, but that browser only supports adding search engines via OpenSearch.

If you need to see manual verification of OpenSearch working before blindly merging this PR, react with a 👀 and I'll try to set this up in a fork and provide before/after screen recordings or something.