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/client-application/src/connection_loop.rs

@@ -0,0 +1,42 @@
+use client_domain::NetworkPort;
+use protocol::{ServerMessage, decode_server_message};
+use std::thread;
+use std::time::Duration;
+
+pub fn run_connection_loop<N: NetworkPort>(
+    net: &mut N,
+    server_addr: &str,
+    poll_interval: Duration,
+    reconnect_delay: Duration,
+    mut on_message: impl FnMut(ServerMessage),
+    mut on_connection_change: impl FnMut(bool),
+) {
+    loop {
+        if !net.is_connected() {
+            match net.connect(server_addr) {
+                Ok(()) => on_connection_change(true),
+                Err(_) => {
+                    on_connection_change(false);
+                    thread::sleep(reconnect_delay);
+                    continue;
+                }
+            }
+        }
+
+        match net.receive() {
+            Ok(Some(payload)) => {
+                if let Ok(msg) = decode_server_message(&payload) {
+                    on_message(msg);
+                }
+            }
+            Ok(None) => {
+                thread::sleep(poll_interval);
+            }
+            Err(_) => {
+                let _ = net.disconnect();
+                on_connection_change(false);
+                thread::sleep(reconnect_delay);
+            }
+        }
+    }
+}