refactor: extract device locale helpers

[ethanpschoen]

Description of this change

Extracts the raw device-locale lookup and a BCP-47 formatter (getDeviceLanguageTag) into their own module, replacing the module-level deviceLanguage const in KetchServiceProvider. Pure refactor, no behavior change.

Part 1/5 of a split of #126 into a stack of small, independently reviewable PRs. Full stack: #137 → #138 → #139 → #140 → #141.

Why is this change being made?

• Chore (non-functional changes)
• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How was this tested? How can the reviewer verify your testing?

npm run lint, npm run typecheck, and npm test (this PR's own deviceLocale.test.ts, 4/4 passing) all pass standalone on this branch.

The full 5-PR stack was verified as a faithful, complete split of #126 before publishing: git diff --exit-code <top-of-stack> origin/ethanpschoen/feat/headless-api is empty (exit 0), and the top of the stack passes the full local gate (lint, typecheck, build, npm test).

Related issues

Refs #126.

Checklist

• I have added tests to cover my changes.
• All new and existing tests passed.
• I have evaluated the security impact of this change, and OWASP Secure Coding Practices have been observed.
• I have informed stakeholders of my changes.