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-next/next-env.d.ts

@@ -0,0 +1,7 @@
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+import "./.next/types/routes.d.ts";
+import "./.next/types/root-params.d.ts";
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.