Add embedded web chat client (closes #7) (#8)

schema/commands/PING.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/PING.json",
+  "title": "PING",
+  "description": "Keepalive. C2S or S2S. Server responds with PONG. RFC 1459 §4.6.2.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "PING" },
+    "body": {
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "Opaque token to be echoed in PONG (single-element array).",
+      "maxItems": 1
+    }
+  },
+  "required": ["command"],
+  "examples": [
+    { "command": "PING", "body": ["1707580000"] }
+  ]
+}