fix: validate the stored language before using it (detectLang)

[Mubashirrrr]

What

detectLang() validates the URL-hash language but trusts the localStorage value unchecked, so a stale/invalid stored value leaves the page silently stuck in English.

Why

const hash = location.hash.replace('#', '').toLowerCase();
if (['en', 'ko', 'ja'].includes(hash)) return hash;   // validated
const stored = localStorage.getItem('harness-lang');
if (stored) return stored;                              // NOT validated

If harness-lang ever holds a value outside en|ko|ja (a future rename, a stale key, etc.), detectLang returns it, and setLang then fails every i18n[key][lang] lookup — leaving all text in English with no active language button — instead of falling through to the navigator.language / 'en' default.

Fix

Validate the stored value against the same allowed set as the hash path:

if (stored && ['en', 'ko', 'ja'].includes(stored)) return stored;

Small consistency/robustness fix; makes both code paths agree.

🤖 Generated with Claude Code