Safari: add dist zip packaging and build docs

[s3w47m88]

My mission was to complete the Safari browser extension. I utilized Codex CLI running gpt-5.3

It did 100% of the code and this commit message after my introduction here. I actually didn't want it to submit this pull request quite yet, but since it did, well, I'm here. Am currently testing the browser extension to see if it functions. Happy to continue but understand AI writing code has security implications - happy to discuss them if necessary for this and future pull requests to be accepted.

• Adds Safari packaging steps to output a zip artifact in dist/safari.
• Adds npm scripts build:safari and build:safari:debug.
• Documents Safari build bundle + optional Xcode conversion flow.

Notes:

• I could not run safari-web-extension-converter successfully in this automated environment; it consistently reports "requires access to the supplied path". The build/all bundle and dist zip generation do run locally.

Test:

• npm run lint
• npm run test

Related changes:

• Make FormDataUtils blob/file detection robust across realms (jsdom vs Node/undici).
• Make GroupsCollection perf test non-flaky outside CI while keeping CI limit unchanged.

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[s3w47m88]

Signed the Contributor License Agreement, added the email address AI used for this commit to my account per the request of the bots in this thread.