feat: store language code

[overskul]

store language code in local storage

[greptile-apps]

Greptile Summary

This PR stores the selected language code in browser storage. The main changes are:

• Lowercase and resolve the requested language code.
• Fall back to en-us for unsupported codes.
• Save the resolved code to window.localStorage["language"] after loading strings.

Confidence Score: 4/5

The language persistence path can save stale or inconsistent state during settings changes.

• Fast language changes can persist an older selection after the newer one.
• Storage write failures can leave settings, strings, and stored language out of sync.
• Unsupported browser language codes are normalized to the fallback before being stored.

src/lib/lang.js

Important Files Changed

Filename	Overview
src/lib/lang.js	Adds localStorage persistence to language setup, with async ordering and storage-error risks in the settings update flow.

_{Reviews (1): Last reviewed commit: "feat: store language code" | Re-trigger Greptile}

[greptile-apps]

Stale Language Wins Race

When settings.applyLangSetting() fires lang.set(value) without awaiting it, two quick language changes can resolve out of order. The older lang.strings() import can finish last and this line then persists the stale code, so the next startup can restore a language different from the user's latest selection.

[greptile-apps]

Storage Failure Leaves Split State

When storage is unavailable or throws, this new write rejects after window.strings has already been updated. The settings-change path does not await lang.set(value), so the saved setting, in-memory strings, and persisted language value can diverge without the caller seeing the failure.

[greptile-apps]

Fallback Overwrites Requested Code

For an unsupported input such as a fresh-install navigator.language value of en or es, the fallback expression rewrites code to en-us before the new persistence step. That stores the fallback instead of the requested language code, so anything reading localStorage["language"] loses the original browser/user language value.