Flutter client posts FlutterTimezone.getLocalTimezone() to
PUT /api/me/timezone on every setSession (login / register success)
and on every AuthController.build (app cold-start with valid
session), when the locally-stored tz_last_sent_at is >7 days old.
Cadence tracked in flutter_secure_storage so it survives app
restarts.
Failures swallowed: the server's UTC default + last-known value
keep the scheduler functioning until the next attempt.
Completes the client side of #392 Half B (per-user timezone
scheduling).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
AuthController is an AsyncNotifier holding the currently-logged-in
User. server_url, session_token, current_user all live in
flutter_secure_storage. ServerUrlScreen probes /healthz before saving
the URL so we don't store a bogus base.
bvandeusen