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

schema/commands/TOPIC.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/TOPIC.json",
+  "title": "TOPIC",
+  "description": "Get or set channel topic. C2S: set topic (body present) or query (body absent). S2C: topic change notification. RFC 1459 §4.2.4.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "TOPIC" },
+    "from": { "type": "string", "description": "Nick that changed the topic (S2C)." },
+    "to": { "type": "string", "description": "Channel name.", "pattern": "^#[a-zA-Z0-9_-]+$" },
+    "body": {
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "New topic text (single-element array). Empty array clears the topic.",
+      "maxItems": 1
+    }
+  },
+  "required": ["command", "to"],
+  "examples": [
+    { "command": "TOPIC", "from": "alice", "to": "#general", "body": ["Welcome to the chat"] }
+  ]
+}