The first-run consent banner had no link to a privacy policy, an
affirmative button ("Help improve") that didn't read as a consent
choice, and a fixed bottom-right card that crowded content on phones.
- Add a "Read the privacy policy" link (external-link icon, accent,
underlined) above the actions, plus a root PRIVACY.md it points to
documenting the telemetry behaviour the modal discloses.
- Rename the CTAs to "Share usage data" / "Don't share" so both name
the action; they stay equal-prominence per the EDPB/GDPR comment.
- Stretch the banner to a bottom-edge bar under 540px with a
safe-area inset so it clears mobile browser chrome.
- Add PrivacyConsentModal tests; sync the new i18n key to every
locale and update the consent-label assertion in App.connectors.
Refs #1756
