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/characterLoader.js

@@ -0,0 +1,26 @@
+// characterLoader.js - Loads character spritesheets from server and registers them
+import { registerCharacter } from './sprites.js';
+
+/**
+ * Fetch character data from the server and register all characters
+ * with the sprite system so NPCs can use them.
+ * @returns {Promise<number>} number of characters loaded
+ */
+export async function loadCharacters() {
+    try {
+        const res = await fetch('/api/characters');
+        const data = await res.json();
+        if (!data.characters) return 0;
+
+        const entries = Object.entries(data.characters);
+        const promises = entries.map(([id, c]) =>
+            registerCharacter(id, c.name, c.spritesheet, c.frameW, c.frameH)
+        );
+        await Promise.all(promises);
+        console.log(`[characterLoader] loaded ${entries.length} character(s)`);
+        return entries.length;
+    } catch (e) {
+        console.warn('[characterLoader] failed to load characters:', e);
+        return 0;
+    }
+}