fix: HTTPS/WebSocket mixed-content — add PHP SSE relay for scale gateway

The browser (HTTPS) cannot connect to ws:// directly (mixed-content block).
Solution: PHP SSE relay bridges the gap server-side.

- api/scale_relay.php: GET ?url=ws://ip:port
  PHP opens WS connection to Android gateway, streams JSON frames as
  Server-Sent Events to the browser over existing HTTPS connection.
  Includes WS handshake, masked client frames, frame decoder, keep-alive.

- api/scale_ping.php: GET ?url=ws://ip:port
  One-shot connectivity test, returns {"ok":true} or {"ok":false,"error"}.

- app.js: WebSocket -> EventSource (SSE)
  _scaleWs -> _scaleEs, connects to /api/scale_relay.php
  testScaleConnection() -> fetch /api/scale_ping.php (no more direct ws://)
  readScaleWeight(): removed get_weight send (scale streams continuously)

api/scale_ping.php

@@ -0,0 +1,67 @@
+<?php
+/**
+ * EverShelf Scale Gateway — Connection ping / test
+ *
+ * Performs a WebSocket handshake with the gateway and returns
+ * {"ok":true} on success, {"ok":false,"error":"..."} on failure.
+ *
+ * Usage: GET /api/scale_ping.php?url=ws%3A%2F%2F192.168.1.100%3A8765
+ */
+
+header('Content-Type: application/json');
+header('Cache-Control: no-cache');
+
+$rawUrl = $_GET['url'] ?? '';
+
+if (!preg_match('#^ws://[0-9a-zA-Z][\w.\-]*(:\d{1,5})?(/.*)?$#', $rawUrl)) {
+    echo json_encode(['ok' => false, 'error' => 'Invalid gateway URL (must start with ws://)']);
+    exit;
+}
+
+$parsed = parse_url($rawUrl);
+$host   = $parsed['host'] ?? '';
+$port   = (int)($parsed['port'] ?? 8765);
+$path   = ($parsed['path'] ?? '') ?: '/';
+
+if (!$host || $port < 1 || $port > 65535) {
+    echo json_encode(['ok' => false, 'error' => 'Invalid host or port']);
+    exit;
+}
+
+// Try to open a TCP connection with a 5-second timeout
+$sock = @stream_socket_client("tcp://{$host}:{$port}", $errno, $errstr, 5);
+if (!$sock) {
+    echo json_encode(['ok' => false, 'error' => "Cannot connect to {$host}:{$port} — {$errstr}"]);
+    exit;
+}
+
+stream_set_timeout($sock, 5);
+
+// Perform WebSocket handshake
+$wsKey = base64_encode(random_bytes(16));
+fwrite($sock,
+    "GET {$path} HTTP/1.1\r\n" .
+    "Host: {$host}:{$port}\r\n" .
+    "Upgrade: websocket\r\n" .
+    "Connection: Upgrade\r\n" .
+    "Sec-WebSocket-Key: {$wsKey}\r\n" .
+    "Sec-WebSocket-Version: 13\r\n" .
+    "\r\n"
+);
+
+// Read HTTP response (looking for 101 Switching Protocols)
+$resp = '';
+while (!feof($sock)) {
+    $line = fgets($sock, 1024);
+    if ($line === false) break;
+    $resp .= $line;
+    if ($line === "\r\n") break;
+}
+
+fclose($sock);
+
+if (str_contains($resp, '101')) {
+    echo json_encode(['ok' => true]);
+} else {
+    echo json_encode(['ok' => false, 'error' => 'WebSocket handshake failed — check that the gateway app is running']);
+}