Google sign-in and account linking

Adds Google sign-in with installation-specific client credentials stored outside the public directory.
Existing users explicitly connect or disconnect Google in their profile after confirming their SkyMission password.
New Google users complete the normal registration, password and current terms; registration controls and local roles remain authoritative.
Verifies signed ID tokens, issuer, audience, expiry and nonce; uses single-use state and PKCE for authorization code exchange.
Retains lightweight automatic update backups.

Changelog