[Refactor] settingsStore clamps language to 24 chars without validating against allowed locales

[RUKAYAT-CODER]

Overview

src/lib/settings/store.ts clamps the language setting to 24 characters as a rough guard but does not validate the value against the SUPPORTED_LANGUAGES list in src/locales/config.ts. An unsupported locale like zz-ZZ (24 chars) passes the check, gets stored, and causes translationManager to fail to load translations, falling back to English silently.

Specifications

Features:

• Language settings are validated against the SUPPORTED_LANGUAGES allowlist before being stored
• Unsupported locales are silently replaced with the default language

Tasks:

• Import SUPPORTED_LANGUAGES from src/locales/config.ts into settingsStore
• In patchSettings, validate language: if not in SUPPORTED_LANGUAGES, fall back to DEFAULT_LANGUAGE
• Remove the 24-character clamp since the allowlist check is sufficient
• Add a test asserting an unsupported locale is replaced with en

Impacted Files:

• src/lib/settings/store.ts
• src/locales/config.ts

Acceptance Criteria

• Setting language to an unsupported locale stores the default language instead
• Setting language to a valid locale stores it correctly
• The 24-character length clamp is removed