Flutter tests base: Riverpod harness, routing, critical notifiers, DESIGN_SYSTEM goldens

[ermeme]

Milestone: 2 — CI / infra base
Type: tests
Priority: high

test/ contains only the default widget_test.dart. There is no scaffolding for testing Riverpod state, no router tests, no goldens.

Scope

• Add a small test harness for ProviderContainer + overrides (factories for IdentityService, nostr_api, common providers).
• Tests for critical notifiers:
  • BackupReminderNotifier
  • FirstRunNotifier
  • OrderBookProvider + filters
  • Trade state providers
• Golden tests for the DESIGN_SYSTEM tokens (order card, status chips, role badges, notification card) — both themes.
• Wire the suite into the CI workflow.