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>

Dockerfile

@@ -7,5 +7,6 @@ COPY editor.html public/
 COPY css/ public/css/
 COPY js/ public/js/
 COPY assets/ public/assets/
+RUN mkdir -p public/data
 EXPOSE 80
 CMD ["node", "server.js"]