fix(frontend): connect Settings DisplaySection language selector to next-intl routing (#2010)

[aazminu]

Summary

Fixes #2010
Closes #2010
/claim #2010

• Connect DisplaySection's language selector in frontend/src/app/[locale]/settings/page.tsx to useLocale() and useRouter() / usePathname() (matching LanguageSwitcher.tsx) so selecting a language updates the active next-intl locale route.
• Restrict the language selector options to the supported locales (en, es, tl) and remove the outdated fallback warning banner.
• Update frontend/src/app/[locale]/settings/page.test.tsx to verify the supported locale options (en, es, tl), absence of the stale fallback banner, and router.replace locale navigation.

Settlement Details

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