fix: dashboard errors — Math.abs, API routing, fetchHistorical guard

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

dashboard/static/vbt.html

@@ -236,7 +236,7 @@ function renderDetail(r){
   const wr=(r.win_rate??0)*100,pf=r.profit_factor??0,eq=r.end_equity??10000;
   const tr=r.total_trades??0,nb=r.n_bars??0,so=r.sortino??0;
   const asset=r.asset||'-';const params=r.params||{};
-  const calmar=r.calmar||((dd)?(ret/abs(dd)):0);
+  const calmar=r.calmar||((dd)?(ret/Math.abs(dd)):0);
   const expec=r.expectancy??(tr?(r.pnl??0)/tr:0);
 
   let avgDur='';