chore/feat: frontend API DRYing

[akx]

Pull Request Checklist

• Target branch: Pull requests should target the dev branch.
• Description: Briefly describe the changes in this pull request.
• Changelog: Ensure a changelog entry following the format of Keep a Changelog is added at the bottom of the PR description.
• Testing: Have you written and run sufficient tests for the changes?
• Code Review: Have you self-reviewed your code and addressed any coding standard issues?

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Description

This PR is a continuation/rework of #2227.

It refactors most frontend API calls into a couple of idiomatic helpers wherever possible. As discussed in #2227's comments, while the helpers are generic over the expected API return type, they're purposefully loose for the time being; users may opt-in to doing do...Request<T>.

Notes/Code Review

The main refactoring commit was done manually; I tried to be careful that the calls are equivalent to what the preimage was, but of course something could have slipped past these non-AI eyeballs.

Testing

Since there are no automated tests for the backend, I did some manual testing.

I tested that the app seems to work (signup, signin, ollama chat, llava "what's in this image", tagging, etc.).

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Changelog Entry

Changed

• Refactored frontend API calls and error handling to reduce repetition.

[akx]

Btw, I can split this into multiple PRs that refactor a couple of files at a time if it'd make things easier to review ✌️

[akx]

@tjbck Rebased. CI errors seem unrelated.