Adds a centralized BackendReachability singleton fed by a Dio
interceptor (connection-level failures only) and connectivity_plus.
PolledValue subscribes to it and pauses on offline / reloads
immediately on reconnect, replacing the previous per-card error
cascade. MainShell renders a global OfflineBanner with Retry and
Settings actions; cards downgrade error→stale when offline so the
banner is the only red element.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Implements the /about route with app description, version info,
clickable links to the source repo and AGPL-3.0 license, and
third-party copyright notices. Adds url_launcher for cross-platform
link handling.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
