feat(flutter): report library import results as toasts

[darwin67]

Summary

Layer 3 of the Shadcn stack (base: the notice infrastructure). Moves transient import results off the persistent error banner and onto Sonner toasts.

• _import now raises a Notice for every outcome:
  • clean success → success toast (Imported N books.)
  • partial / warning / cancelled → destructive toast (the existing summary text)
  • hard failure → destructive toast
• Import no longer sets error, so LibraryFailure.import is no longer needed for the retry banner; _libraryController tests now assert model.notice and its kind.
• Persistent conditions (provider cleanup, managed-deletion debt, load failures) intentionally remain banners — see the stack notes.

Testing

• Updated the import-state-machine tests to assert notices.
• make test-flutter (284), make lint-flutter, make check-fmt-flutter.

Stack

1. brand theme + reader theme consolidation (feat(flutter): apply the Shosai brand accent and unify reader theming on Shadcn #109)
2. notice/Sonner infrastructure (feat(flutter): present controller notices as Sonner toasts #110)
3. this PR — import results → toasts
4. managed-deletion / retry-bearing conditions (toast vs. persistent surface)
5. reader transient errors → toasts
6. ShadSheet/ShadTabs responsive surfaces
7. ShadContextMenu/ShadBadge/ShadSeparator
8. reader/dialog goldens + boundary docs