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/tools/hand-tool.js

@@ -0,0 +1,34 @@
+export const createHandTool = ({ viewport }) => {
+  let isDragging = false;
+  let dragStartX = 0;
+  let dragStartY = 0;
+  let scrollStartLeft = 0;
+  let scrollStartTop = 0;
+
+  return {
+    name: "hand",
+    cursor: "grab",
+    onMouseDown: (event) => {
+      isDragging = true;
+      dragStartX = event.clientX;
+      dragStartY = event.clientY;
+      scrollStartLeft = viewport.scrollLeft;
+      scrollStartTop = viewport.scrollTop;
+      viewport.style.cursor = "grabbing";
+      event.preventDefault();
+    },
+    onMouseMove: (event) => {
+      if (!isDragging) return;
+      viewport.scrollLeft = scrollStartLeft - (event.clientX - dragStartX);
+      viewport.scrollTop = scrollStartTop - (event.clientY - dragStartY);
+    },
+    onMouseUp: () => {
+      if (!isDragging) return;
+      isDragging = false;
+      viewport.style.cursor = "grab";
+    },
+    onDeactivate: () => {
+      isDragging = false;
+    },
+  };
+};