add root route: redirect / → /admin (default surface)

The chat surface (which owned /) was gutted, leaving no root route.
Now / redirects to /admin as the default surface. Configurable
default surface planned for v0.2.0.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/main.go

@@ -679,6 +679,11 @@ func main() {
 	pages.SetVersion(Version)
 	pageEngine := pages.New(cfg, stores)
 
+	// Root redirect → default surface (admin for now, configurable in v0.2.0)
+	base.GET("/", func(c *gin.Context) {
+		c.Redirect(http.StatusTemporaryRedirect, cfg.BasePath+"/admin")
+	})
+
 	// Login page — no auth required
 	base.GET("/login", pageEngine.RenderLogin())