feat(inventory): auto-recover from a transient stream failure

Instead of showing a retry button on the first failure, silently re-open
the inventory stream with exponential backoff (250ms→4s, 6 attempts,
staying in the loading spinner). The startup DB-not-ready race clears in
well under a second, so the user just sees the spinner briefly and the
list appears — no tap needed. Only after the retry budget is exhausted do
we surface the manual-retry error.

Tests cover auto-recovery, no error flicker during retries, and the
give-up path (via fake_async to fast-forward the backoff).

apps/app_seeds/pubspec.yaml

@@ -86,6 +86,8 @@ dev_dependencies:
     sdk: flutter
 
   flutter_lints: ^6.0.0
+  # Virtual clock to fast-forward the inventory auto-retry backoff in tests.
+  fake_async: ^1.3.1
   build_runner: ^2.4.13
   drift_dev: ^2.28.0
   slang_build_runner: ^4.7.0