arch: split ConfigRepository, extract polling, consolidate conversions, decouple protocol

- Value↔JSON: From impls on domain Value behind `json` feature, delete 4 duplicate converters
- ConfigRepository split into ConfigRepository (12), UserRepository (3), WidgetStateCache (2)
- polling orchestration moved from bootstrap to application::polling_service
- WidgetRenderer in client-domain owns scroll/cache, both clients use it
- network loop consolidated into client-application::run_connection_loop
- protocol crate drops domain dep, Wire↔Domain conversions move to adapters

crates/domain/Cargo.toml

@@ -3,6 +3,10 @@ name = "domain"
 version = "0.1.0"
 edition = "2024"
 
+[features]
+json = ["serde_json"]
+
 [dependencies]
+serde_json = { workspace = true, optional = true }
 
 [dev-dependencies]