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>

server.js

@@ -8,6 +8,7 @@ const path = require('path');
 const PORT = process.env.PORT || 80;
 const STATIC_DIR = path.join(__dirname, 'public');
 const MAPS_FILE = path.join(STATIC_DIR, 'js', 'world', 'maps.js');
+const CHARS_FILE = path.join(STATIC_DIR, 'data', 'characters.json');
 
 const MIME = {
     '.html': 'text/html',
@@ -92,6 +93,43 @@ const server = http.createServer((req, res) => {
         return;
     }
 
+    // === API: GET /api/characters — read character data ===
+    if (req.method === 'GET' && req.url === '/api/characters') {
+        fs.readFile(CHARS_FILE, 'utf-8', (err, data) => {
+            if (err) {
+                res.writeHead(200, { 'Content-Type': 'application/json' });
+                res.end(JSON.stringify({ characters: {} }));
+                return;
+            }
+            res.writeHead(200, { 'Content-Type': 'application/json' });
+            res.end(data);
+        });
+        return;
+    }
+
+    // === API: PUT /api/characters — write character data ===
+    if (req.method === 'PUT' && req.url === '/api/characters') {
+        let body = '';
+        req.on('data', chunk => { body += chunk; });
+        req.on('end', () => {
+            try {
+                const parsed = JSON.parse(body);
+                // Ensure data dir exists
+                const dataDir = path.dirname(CHARS_FILE);
+                if (!fs.existsSync(dataDir)) fs.mkdirSync(dataDir, { recursive: true });
+                fs.writeFileSync(CHARS_FILE, JSON.stringify(parsed, null, 2), 'utf-8');
+                const count = Object.keys(parsed.characters || {}).length;
+                console.log(`[server] characters.json saved (${count} characters)`);
+                res.writeHead(200, { 'Content-Type': 'application/json' });
+                res.end(JSON.stringify({ ok: true, count }));
+            } catch (e) {
+                res.writeHead(400, { 'Content-Type': 'application/json' });
+                res.end(JSON.stringify({ error: e.message }));
+            }
+        });
+        return;
+    }
+
     // === Static file serving ===
     let filePath = path.join(STATIC_DIR, req.url === '/' ? 'index.html' : req.url);
     // Prevent directory traversal