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/adapters/config-memory/src/lib.rs

@@ -1,6 +1,6 @@
 use domain::{
     ConfigRepository, DataSource, DataSourceId, Layout, LayoutPreset, LayoutPresetId, ThemeConfig,
-    User, WidgetConfig, WidgetId, WidgetState,
+    User, UserRepository, WidgetConfig, WidgetId, WidgetState, WidgetStateCache,
 };
 use std::collections::HashMap;
 use std::sync::RwLock;
@@ -177,6 +177,10 @@ impl ConfigRepository for MemoryConfigStore {
         guard.remove(&id);
         Ok(())
     }
+}
+
+impl UserRepository for MemoryConfigStore {
+    type Error = MemoryConfigError;
 
     async fn get_user_by_username(&self, username: &str) -> Result<Option<User>, Self::Error> {
         let guard = self
@@ -203,6 +207,10 @@ impl ConfigRepository for MemoryConfigStore {
             .map_err(|_| MemoryConfigError::LockPoisoned)?;
         Ok(guard.len() as u32)
     }
+}
+
+impl WidgetStateCache for MemoryConfigStore {
+    type Error = MemoryConfigError;
 
     async fn save_widget_states(
         &self,