Switch remote deploy to vendored source builds

Move remote deployment to a vendored source bundle built on the target host via Docker so redeploys no longer require local cross-compilation or host Go installation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

vendor/github.com/gorilla/websocket/mask_safe.go

@@ -0,0 +1,16 @@
+// Copyright 2016 The Gorilla WebSocket Authors. All rights reserved.  Use of
+// this source code is governed by a BSD-style license that can be found in the
+// LICENSE file.
+
+//go:build appengine
+// +build appengine
+
+package websocket
+
+func maskBytes(key [4]byte, pos int, b []byte) int {
+	for i := range b {
+		b[i] ^= key[pos&3]
+		pos++
+	}
+	return pos & 3
+}