Replace from-scratch rewrite with real ngoduykhanh/wireguard-ui fork

The from-scratch Go rewrite had unresolved bugs (missing go.sum, UI
404s, path issues) from being built without a working local Go
toolchain to verify against. Switching strategy: use the actual
upstream wireguard-ui codebase (proven, battle-tested single-server
manager) as the base, and extend it for multi-server support instead
of re-deriving everything from zero.

Kept our own installers (bootstrap.sh, update.sh, scripts/install.sh,
scripts/proxmox-install.sh) - these still apply, just need updating
to build/install the upstream module layout instead of the old
cmd/wireguard-ui-multi structure.

Module path intentionally left as upstream's own
(github.com/ngoduykhanh/wireguard-ui) for now to avoid touching every
internal import; revisit if this needs to be fully rebranded.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

handler/middlewares.go

@@ -0,0 +1,20 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/labstack/echo/v4"
+)
+
+// ContentTypeJson checks that the requests have the Content-Type header set to "application/json".
+// This helps against CSRF attacks.
+func ContentTypeJson(next echo.HandlerFunc) echo.HandlerFunc {
+	return func(c echo.Context) error {
+		contentType := c.Request().Header.Get("Content-Type")
+		if contentType != "application/json" {
+			return c.JSON(http.StatusBadRequest, jsonHTTPResponse{false, "Only JSON allowed"})
+		}
+
+		return next(c)
+	}
+}