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

cli.py

@@ -215,8 +215,10 @@ def cmd_run(args):
     import asyncio
     from live.node_v2 import ProductionNode
 
+    coins = [c.strip().upper() for c in args.coins.split(",") if c.strip()]
+
     node = ProductionNode(
-        coins=args.coins,
+        coins=coins,
         testnet=not args.mainnet,
         mode=args.mode,
         max_position_per_coin=args.max_position,
@@ -277,7 +279,7 @@ def main():
 
     # run
     pr = sp.add_parser("run", help="Start production node")
-    pr.add_argument("--coins", nargs="+", default=["BTC", "ETH"])
+    pr.add_argument("--coins", default="BTC,ETH", help="Comma-separated coin list")
     pr.add_argument("--mainnet", action="store_true")
     pr.add_argument("--mode", default="paper", choices=["paper", "live"])
     pr.add_argument("--max-position", type=float, default=0.003)