Fixes three bugs in dashboard:
1. vbt.html: bare abs(dd) → Math.abs(dd) — fixes ReferenceError
2. server.py: Add /cv/ routing for Next.js quant dashboard
- Mount _next static assets at /cv/_next (not /cv/ which eats API routes)
- Add /cv/api/* routes for backtests/historical, detail, recalc, risk
- Add /cv/ws WebSocket endpoints for live/paper metrics
- Add /cv/ catchall for Next.js HTML pages
3. dashboard-next/src/lib/api.ts: add res.ok guard to fetchHistorical()
— prevents SyntaxError when API returns HTML error pages
4. sim/maker.py: guard observe() against zero mid_price
ramseshk