feat: unified NOFXi — one system, one port, one UI

Key changes:
- Agent API routes (/api/agent/*) registered directly on NOFX's :8080 server
- No more separate :8900 port — everything on :8080
- AgentChatPage.tsx added to React frontend
- Agent tab (🤖 Agent) added to navigation bar, set as DEFAULT page
- Branding: NOFX → NOFXi everywhere (title, header, branding.ts, i18n)
- Default landing page is now Agent chat, not competition

This is THE change: there is no more 'NOFX' vs 'NOFXi'.
Users open localhost:8080, they see NOFXi.
Agent chat is the first thing they see.
All other pages (Traders, Dashboard, Strategy, Data) are still there.

One binary. One port. One product. NOFXi.

api/agent_routes.go

@@ -0,0 +1,15 @@
+package api
+
+import (
+	"nofx/agent"
+
+	"github.com/gin-gonic/gin"
+)
+
+// RegisterAgentHandler registers NOFXi agent API routes on the main router.
+func (s *Server) RegisterAgentHandler(h *agent.WebHandler) {
+	s.router.POST("/api/agent/chat", gin.WrapF(h.HandleChat))
+	s.router.GET("/api/agent/health", gin.WrapF(h.HandleHealth))
+	s.router.GET("/api/agent/klines", gin.WrapF(h.HandleKlines))
+	s.router.GET("/api/agent/ticker", gin.WrapF(h.HandleTicker))
+}