feat: character/NPC management system with spritesheet support

Add drag & drop spritesheet upload in editor, character registry in
sprites.js, character selector for NPCs, sprite rendering on editor
canvas, server API for character persistence, and game-side character
loading via characterLoader.js.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/world/gameMode.js

@@ -5,6 +5,7 @@ import { initWorldInput, destroyWorldInput, setInteractionHandler } from './worl
 import { getMap } from './maps.js';
 import { saveGadget, openBackpack, getGadgets, openNamingScreen, showNotification } from './inventory.js';
 import { openWiringPanel } from './wiringPanel.js';
+import { loadCharacters } from './characterLoader.js';
 
 // Circuit editor stop function (to stop its render loop when switching modes)
 import { stopCircuitLoop } from '../renderer.js';
@@ -28,7 +29,10 @@ export function registerCircuitEditor(initFn, destroyFn) {
 /**
  * Boot the game — start in world mode
  */
-export function startGame() {
+export async function startGame() {
+    // Load character spritesheets before entering world
+    await loadCharacters();
+
     // Set spawn
     const map = getMap(worldState.currentMap);
     if (map && map.spawn) {