Add UI improvements + refactor frontend to event-driven architecture

Features: disabled place button during cooldown, beep on cooldown end,
hand/paint mode toggle with Space shortcut, soldier count pop animation.

Architecture: centralized DOM refs, AppState store replacing localStorage
polling, event bus decoupling socket from handlers, keyboard manager,
tool manager with Strategy pattern (paint-tool, hand-tool).

painter-js/src/domain/app-state.js

@@ -0,0 +1,20 @@
+export const createAppState = (initial) => {
+  const state = { ...initial };
+  const listeners = {};
+
+  return {
+    get: (key) => state[key],
+    set: (key, value) => {
+      const old = state[key];
+      state[key] = value;
+      (listeners[key] || []).forEach((fn) => fn(value, old));
+    },
+    subscribe: (key, fn) => {
+      if (!listeners[key]) listeners[key] = [];
+      listeners[key].push(fn);
+      return () => {
+        listeners[key] = listeners[key].filter((f) => f !== fn);
+      };
+    },
+  };
+};