Adds a visible warning in the Settings page when `token_store` has
landed in the file fallback instead of the OS keychain. Without this,
a user on a keychain-less system would silently lose the security
benefit introduced in #78 and never know.
- New `get_token_store_mode` service wrapper in authService.ts.
- New `TokenStoreFallbackBanner` component: fetches the mode on mount,
renders nothing when mode is `keychain` or null, renders an
amber warning card when mode is `file`.
- Mounted in SettingsPage right after AccountCard so it sits next to
the account state the user can fix (log out + log back in once the
keychain is available).
- i18n keys under `account.tokenStore.fallback.*` in fr/en.
Refs #66
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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