fix(i18n): Cache initial locale to prevent repeated warnings on page load

[Copilot]

Page load triggered excessive [i18n] Saved locale 'ja' is not enabled. Trying related locales... warnings because getSavedLocale() was called multiple times before IndexedDB/extension manager was ready, and each call logged when enabled locales set was empty.

Changes

• Cache initial locale calculation: Module-level cachedInitialLocale ensures getSavedLocale() and detectBrowserLocale() run only once
• Skip validation when extensions not ready: Early return when enabledLocales.size === 0 — trust saved/browser locale until language packs load; extension change listener handles validation afterward

// Before: ran every render, warned every time
const initialLocale = getSavedLocale() || detectBrowserLocale() || ...

// After: cached, no warnings until extensions ready
if (cachedInitialLocale === null) {
  cachedInitialLocale = getSavedLocale() || detectBrowserLocale() || ...
}

Original prompt

184: [i18n] Saved locale 'ja' is not enabled. Trying related locales...このように、ページを開いたときに、ものすごい数のtryが走る。確かに、ページを開いた瞬間はIndexedDBはまだ準備できてないから、しょうがないんだけど。もうちょい改善できんかな？

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