docs(flutter): document the Shadcn boundary, notices, and Rust enum mapping

[darwin67]

Summary

Final layer of the Shadcn stack (base: the accents). Documentation only.

Adds to docs/flutter-architecture.md:

• The Shadcn / Material boundary: which widgets stay Material (Scaffold, AppBar, CircularProgressIndicator, MaterialApp interop) and why ShadIconAction keeps the Material Tooltip (ShadTooltip has no tooltip semantics).
• Theme ownership (shosaiShadTheme, shosaiReaderShadTheme, reader page colors from ShadColorScheme).
• The transient-feedback model: controller-raised Notice + SonnerBridge, and when to keep persistent surfaces instead (cleanup/deletion debt, failed content, reading-state errors).
• The Rust enum ↔ Dart sealed class mapping and exhaustiveness guidance.

Note: no new reader/dialog goldens in this layer. The reader's page content is Rust-painted, and the existing golden infrastructure is library-only; reader goldens would need font-loaded, raster-faked harnesses, which I'd rather do as a focused test PR than bundle here.

Stack

1. feat(flutter): apply the Shosai brand accent and unify reader theming on Shadcn #109 brand theme + reader theme consolidation
2. feat(flutter): present controller notices as Sonner toasts #110 notice/Sonner infrastructure
3. feat(flutter): report library import results as toasts #111 import results → toasts
4. feat(flutter): present reader transient failures as toasts #112 reader transient failures → toasts
5. feat(flutter): present the library as Shadcn format tabs #113 library format tabs
6. feat(flutter): add Shadcn badges, separators, and annotation context menus #114 badges / separators / context menus
7. this PR — boundary docs