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/application/tests/support/mod.rs

@@ -1,6 +1,7 @@
 use domain::{
     ConfigRepository, DataSource, DataSourceId, DomainEvent, EventPublisher, Layout, LayoutPreset,
-    LayoutPresetId, ThemeConfig, User, WidgetConfig, WidgetId, WidgetState,
+    LayoutPresetId, ThemeConfig, User, UserRepository, WidgetConfig, WidgetId, WidgetState,
+    WidgetStateCache,
 };
 use std::collections::HashMap;
 use std::sync::Mutex;
@@ -123,6 +124,10 @@ impl ConfigRepository for InMemoryConfigRepository {
         self.presets.lock().unwrap().remove(&id);
         Ok(())
     }
+}
+
+impl UserRepository for InMemoryConfigRepository {
+    type Error = Never;
 
     async fn get_user_by_username(&self, _username: &str) -> Result<Option<User>, Self::Error> {
         Ok(None)
@@ -135,6 +140,10 @@ impl ConfigRepository for InMemoryConfigRepository {
     async fn count_users(&self) -> Result<u32, Self::Error> {
         Ok(0)
     }
+}
+
+impl WidgetStateCache for InMemoryConfigRepository {
+    type Error = Never;
 
     async fn save_widget_states(
         &self,