fix: pass language to Collabora file conversions

[xhon-pelushi]

Summary

• Pass the current user locale to Collabora file conversions as a BCP 47 lang parameter.
• Send conversion options as separate multipart fields (not merged into the file part).
• Add focused PHPUnit coverage for language + multipart option payload.

Fixes #5165

Test plan

• Save-as PDF (or other conversion) with a non-English locale and confirm locale-sensitive formatting uses the right language.
• composer run test:unit for the new conversion tests when PHP tooling is available.

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[elzody]

Thank you for working on this @xhon-pelushi. That you want to work on the richdocuments app means a lot. <3

I do have some feedback that I think is worth considering:

In src/helpers/index.js there is a function called languageToBCP47() that translates the language setting given by Nextcloud into a BCP 47 tag that Collabora can correctly handle. Given that, to fix the problem this PR tries to solve, this logic would likely need to be duplicated in PHP since the /convert-to endpoint's lang field also takes the same BCP 47 language tags. I think it would make more sense to move the logic for that to the PHP side and then serve it to the front-end as an initial state, so the logic only lives in one place.

That said, I have not totally tested this PR yet, but given that it works as intended I am happy to merge it in the meantime and we can create a separate issue for refactoring that bit. But if you want to already take the opportunity to take care of that, that's also fine.

Will give it a proper test soon.