Consolidate dual-server setup into single Express server; fix binary auto-detection

[Copilot]

Two issues: start-all.sh launched both a Vite dev server (5173) and an Express API server (3001), forcing users to juggle two ports. Additionally, .env.example hardcoded placeholder ACESTEP_BIN=/path/to/acestep-generate, triggering a startup warning on every fresh clone even though the server already auto-detects binaries from ./bin/.

Bug fix: APP_ROOT wrong depth

server/src/config/index.ts lived at server/src/config/ but resolved APP_ROOT with '../..', landing at server/ instead of the repo root. Binary detection was silently checking server/bin/ace-qwen3 (never exists) instead of bin/ace-qwen3 (where build.sh installs them).

// Before — resolves to <repo>/server/ (wrong)
const APP_ROOT = path.resolve(__dirname, '../..');

// After — resolves to <repo>/ (correct)
const APP_ROOT = path.resolve(__dirname, '../../..');

Single-server: Express serves the built frontend

Added SPA static serving to server/src/index.ts. index.html is read once at startup and served from memory for all unmatched non-API routes:

const DIST_DIR = path.join(__dirname, '../../dist');
if (existsSync(DIST_DIR)) {
  app.use(express.static(DIST_DIR));
  const indexHtml = readFileSync(path.join(DIST_DIR, 'index.html'), 'utf-8');
  app.get('*', (req, res, next) => {
    if (req.path.startsWith('/api/') || req.path.startsWith('/audio/') || ...) return next();
    res.type('html').send(indexHtml);
  });
}

start-all.sh / start-all.bat

• Runs npm run build before starting the server
• Starts a single Node.js process; no Vite dev server
• Single URL shown: http://localhost:3001
• PID written to logs/server.pid; stop-all.sh handles both new and legacy PID files

Config / env cleanup

• .env.example and server/.env.example: removed ACESTEP_BIN, ACESTEP_MODEL, FRONTEND_URL, VITE_API_URL placeholders — all auto-detected
• server/src/config/index.ts: frontendUrl defaults to http://localhost:<PORT> (same origin); extracted resolvedPort constant to eliminate the duplicated '3001' literal
• setup.sh: "Next steps" no longer instructs users to manually edit .env
• README.md: updated quick start (3 steps, not 4), usage table, config reference, troubleshooting

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