Move admin dashboard to admin.myhoneydue.com subdomain

- Remove Next.js basePath "/admin" — admin now serves at root
- Update all internal links from /admin/xxx to /xxx
- Change Go proxy to host-based routing: admin subdomain requests
  proxy to Next.js, /admin/* redirects to main web app
- Update timeout middleware skipper for admin subdomain

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/router/router.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"os"
 	"strings"
 	"time"
 
@@ -73,7 +74,9 @@ func SetupRouter(deps *Dependencies) *echo.Echo {
 			// timeout middleware wraps the response writer in *http.timeoutWriter
 			// which does not implement http.Flusher, causing a panic when
 			// httputil.ReverseProxy or WebSocket upgraders try to flush.
-			return strings.HasPrefix(path, "/admin") ||
+			// Also skip for admin subdomain (all requests proxied to Next.js).
+			adminHost := os.Getenv("ADMIN_HOST")
+			return (adminHost != "" && c.Request().Host == adminHost) ||
 				strings.HasPrefix(path, "/_next") ||
 				strings.HasSuffix(path, "/ws")
 		},