Address remaining PR #768 review comments for locale safety and selector correctness

[Copilot]

This PR applies the unresolved Copilot review feedback from thread #pullrequestreview-4408060524 in PR #768, with minimal scoped changes. It hardens locale/time formatting paths and fixes edge-case rendering in the profile locale selector.

• Locale provider type and context contract

  • Replaced type LocalePref = "auto" | string with a non-widening union so "auto" remains a real sentinel in type narrowing.
  • Switched LocaleProviderContext to LocaleProviderState | undefined defaulting to undefined, so useLocale() correctly throws outside LocaleProvider instead of silently returning defaults.

• Date/time option precedence

  • Updated withTimeZone to always strip opts.timeZone first, then apply the function parameter.
  • Ensures timeZone argument is authoritative (including explicit "local" semantics) regardless of caller-supplied options.

• Profile locale selector unknown-value handling

  • Added fallback rendering for currently selected locale values not present in LOCALE_TAGS (e.g., historical/manual values), so the Select keeps a visible selected item.

• Focused coverage for new behavior

  • Added tests for:
    • useLocale() throwing outside provider,
    • withTimeZone stripping/overriding existing opts.timeZone,
    • Profile locale selector preserving unknown stored locale values.

// datetime.ts
export function withTimeZone(opts: DateTimeFormatOptions, timeZone?: string): DateTimeFormatOptions {
  const { timeZone: _ignored, ...rest } = opts
  if (!timeZone || timeZone === 'local') return rest
  return { ...rest, timeZone }
}

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✅ fredrik-dahlgren
❌ Copilot
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