feat(converto): add UI for settings

[DevYukine]

Description
^{Explain the changes or enhancements you are proposing with this pull request.}

A lot of design decisions are still open here: how we integrate this, how we want to change the UI for it, and how we want to display metadata like title ids.

This is a draft PR, stacked on rommapp#4337. Frontend only — more will land here before it is ready to merge.

Adds the Conversion settings page for the rom-converto integration: toggles for download conversion and scan metadata extraction, the cache TTL, and per-platform target format selection. Includes the API/store wiring and locales for the new settings.

Disclosure
I am the author of rom-converto. I used GLM 5.3 for this but verified and reviewed the result before submitting.

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes (existing frontend suite still passes)

Screenshots (if applicable)

(will attach once opened)

[DevYukine]

Moved to rommapp#4338 (stacking on a branch of the upstream repo needs write access there, so the frontend PR now lives on the org repo instead).