Preserve explicit model and provider architecture (#30)

AGENTS.md

@@ -8,6 +8,24 @@
 - For cross-module work, apply every relevant scoped instruction file and
   completion workflow.
 
+## Data and state architecture
+
+Preserve explicit **DTO → mapper → service → data model → provider → UI**
+boundaries across the project. This is a standing requirement for application
+features, shared packages, stub services and future mobile clients.
+
+- DTOs describe transport data; mappers convert it to/from domain data models.
+- Services own external I/O and use mappers. Service contracts return data models,
+  never transport DTOs or untyped response maps to the UI.
+- Data models remain distinct from provider state and transport serialization.
+- Providers sit on top of services, coordinating loading, errors and state and
+  exposing models to the UI. Widgets render models and invoke provider actions.
+- Keep reusable domain models, services and providers in shared packages; clients
+  compose them rather than copy business logic or create alternate data paths.
+- Preserve these boundaries when replacing stubs with real backend adapters.
+  Review every changed feature against them before delivery. Apply the detailed
+  frontend requirements in `frontend/AGENTS.md` to all Flutter clients/packages.
+
 ## UX
 
 For any user-facing interface work, follow the