Introduce a headless `flutter test` suite (83 tests) covering models,
utilities, every API endpoint, and the five screens, with shared helpers
and fixtures so new tests stay terse.
API endpoint methods are statically-dispatched extensions on the
BackendAPI singleton, so they cannot be mocked via `implements`. Mock at
the Dio HTTP-adapter layer (http_mock_adapter) instead, enabled by two
small @visibleForTesting seams: BackendAPI.dioForTesting swaps the
singleton's Dio, and BackendReachability.forceOnlineForTesting() forces a
deterministic online state so polling widgets load.
Add frontend/run_tests.sh and document it in CLAUDE.md.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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>
