feat(flutter): present controller notices as Sonner toasts

[darwin67]

Summary

Layer 2 of the Shadcn stack (base: the brand-theme PR). Introduces the Elm-consistent transient-notice boundary and uses it for the first event (reader settings saved).

• shared/notice.dart: Notice/NoticeKind value data (no closures).
• shared/sonner_bridge.dart: SonnerBridge, a view-layer adapter that observes the immutable model, presents a ShadToast exactly once per notice id, and reports back via a consumption callback. It never mutates model state.
• LibraryModel gains a nullable notice; LibraryController raises a success notice on settings save and clears it on LibraryNoticeConsumed.
• ProductShell wraps its scaffold in SonnerBridge.

This layer keeps the existing retry banners untouched; the next layers migrate the banners to toasts.

Testing

• Controller test: settings save raises a success notice and consumption clears it.
• Component test: SonnerBridge presents a notice once and reports consumption.
• 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. this PR — notice/Sonner infrastructure
3. library operation results → toasts (retire banners)
4. library retryable errors → destructive toasts with Retry
5. reader transient errors → toasts
6. ShadSheet/ShadTabs responsive surfaces
7. ShadContextMenu/ShadBadge/ShadSeparator
8. reader/dialog goldens + boundary docs