Add bot-driven room app UI via im.paarrot.ui state.

Bots can publish a declarative panel UI that takes over a room; user
actions are sent as im.paarrot.ui.action timeline events.

docs/handoff/features/room-app-ui/samples/trivia-ui-state.json

@@ -0,0 +1,88 @@
+{
+  "version": 1,
+  "title": "Trivia Night",
+  "css": ".score { font-weight: 700; } .hint { opacity: 0.75; }",
+  "root": {
+    "type": "panel",
+    "id": "main",
+    "children": [
+      {
+        "type": "text",
+        "id": "q",
+        "text": "Capital of France?"
+      },
+      {
+        "type": "text",
+        "id": "hint",
+        "className": "hint",
+        "text": "Pick one answer below."
+      },
+      {
+        "type": "spacer",
+        "id": "sp1",
+        "size": 16
+      },
+      {
+        "type": "row",
+        "id": "answers",
+        "children": [
+          {
+            "type": "button",
+            "id": "a",
+            "label": "Paris",
+            "action": "answer",
+            "value": "paris",
+            "variant": "Primary"
+          },
+          {
+            "type": "button",
+            "id": "b",
+            "label": "Lyon",
+            "action": "answer",
+            "value": "lyon",
+            "variant": "Secondary"
+          },
+          {
+            "type": "button",
+            "id": "c",
+            "label": "Marseille",
+            "action": "answer",
+            "value": "marseille",
+            "variant": "Secondary"
+          }
+        ]
+      },
+      {
+        "type": "spacer",
+        "id": "sp2",
+        "size": 24
+      },
+      {
+        "type": "text",
+        "id": "score_label",
+        "className": "score",
+        "text": "Score: 0"
+      },
+      {
+        "type": "input",
+        "id": "nick",
+        "name": "nickname",
+        "label": "Display name on the board",
+        "placeholder": "Optional nickname"
+      },
+      {
+        "type": "row",
+        "id": "submit_row",
+        "children": [
+          {
+            "type": "button",
+            "id": "join",
+            "label": "Join board",
+            "action": "join",
+            "variant": "Success"
+          }
+        ]
+      }
+    ]
+  }
+}