feat(webui): render video media attachments

Add signed media URLs to live WebSocket replies and teach the WebUI to classify and render video attachments, so bot-sent videos can play inline in both live chats and session history.

Made-with: Cursor

webui/src/tests/useNanobotStream.test.tsx

@@ -92,4 +92,25 @@ describe("useNanobotStream", () => {
     expect(result.current.messages[1].role).toBe("assistant");
     expect(result.current.messages[1].kind).toBeUndefined();
   });
+
+  it("attaches assistant media_urls to complete messages", () => {
+    const fake = fakeClient();
+    const { result } = renderHook(() => useNanobotStream("chat-m", []), {
+      wrapper: wrap(fake.client),
+    });
+
+    act(() => {
+      fake.emit("chat-m", {
+        event: "message",
+        chat_id: "chat-m",
+        text: "video ready",
+        media_urls: [{ url: "/api/media/sig/payload", name: "demo.mp4" }],
+      });
+    });
+
+    expect(result.current.messages).toHaveLength(1);
+    expect(result.current.messages[0].media).toEqual([
+      { kind: "video", url: "/api/media/sig/payload", name: "demo.mp4" },
+    ]);
+  });
 });