refactor(canvas): host A2UI via gateway

apps/android/app/src/main/java/com/steipete/clawdis/node/NodeRuntime.kt

@@ -605,9 +605,17 @@ class NodeRuntime(context: Context) {
         BridgeSession.InvokeResult.ok("""{"format":"${snapshotParams.format.rawValue}","base64":"$base64"}""")
       }
       ClawdisCanvasA2UICommand.Reset.rawValue -> {
-        val ready = ensureA2uiReady()
+        val a2uiUrl = resolveA2uiHostUrl()
+          ?: return BridgeSession.InvokeResult.error(
+            code = "A2UI_HOST_NOT_CONFIGURED",
+            message = "A2UI_HOST_NOT_CONFIGURED: gateway did not advertise canvas host",
+          )
+        val ready = ensureA2uiReady(a2uiUrl)
         if (!ready) {
-          return BridgeSession.InvokeResult.error(code = "A2UI_NOT_READY", message = "A2UI not ready")
+          return BridgeSession.InvokeResult.error(
+            code = "A2UI_HOST_UNAVAILABLE",
+            message = "A2UI host not reachable",
+          )
         }
         val res = canvas.eval(a2uiResetJS)
         BridgeSession.InvokeResult.ok(res)
@@ -619,9 +627,17 @@ class NodeRuntime(context: Context) {
           } catch (err: Throwable) {
             return BridgeSession.InvokeResult.error(code = "INVALID_REQUEST", message = err.message ?: "invalid A2UI payload")
           }
-        val ready = ensureA2uiReady()
+        val a2uiUrl = resolveA2uiHostUrl()
+          ?: return BridgeSession.InvokeResult.error(
+            code = "A2UI_HOST_NOT_CONFIGURED",
+            message = "A2UI_HOST_NOT_CONFIGURED: gateway did not advertise canvas host",
+          )
+        val ready = ensureA2uiReady(a2uiUrl)
         if (!ready) {
-          return BridgeSession.InvokeResult.error(code = "A2UI_NOT_READY", message = "A2UI not ready")
+          return BridgeSession.InvokeResult.error(
+            code = "A2UI_HOST_UNAVAILABLE",
+            message = "A2UI host not reachable",
+          )
         }
         val js = a2uiApplyMessagesJS(messages)
         val res = canvas.eval(js)
@@ -707,7 +723,14 @@ class NodeRuntime(context: Context) {
     return code to "$code: $message"
   }
 
-  private suspend fun ensureA2uiReady(): Boolean {
+  private fun resolveA2uiHostUrl(): String? {
+    val raw = session.currentCanvasHostUrl()?.trim().orEmpty()
+    if (raw.isBlank()) return null
+    val base = raw.trimEnd('/')
+    return "${base}/__clawdis__/a2ui/"
+  }
+
+  private suspend fun ensureA2uiReady(a2uiUrl: String): Boolean {
     try {
       val already = canvas.eval(a2uiReadyCheckJS)
       if (already == "true") return true
@@ -715,8 +738,7 @@ class NodeRuntime(context: Context) {
       // ignore
     }
 
-    // Ensure the default canvas scaffold is loaded; A2UI is now hosted there.
-    canvas.navigate("")
+    canvas.navigate(a2uiUrl)
     repeat(50) {
       try {
         val ready = canvas.eval(a2uiReadyCheckJS)

apps/android/app/src/main/java/com/steipete/clawdis/node/bridge/BridgeSession.kt

@@ -61,6 +61,7 @@ class BridgeSession(
   private val json = Json { ignoreUnknownKeys = true }
   private val writeLock = Mutex()
   private val pending = ConcurrentHashMap<String, CompletableDeferred<RpcResponse>>()
+  @Volatile private var canvasHostUrl: String? = null
 
   private var desired: Pair<BridgeEndpoint, Hello>? = null
   private var job: Job? = null
@@ -77,10 +78,13 @@ class BridgeSession(
     scope.launch(Dispatchers.IO) {
       job?.cancelAndJoin()
       job = null
+      canvasHostUrl = null
       onDisconnected("Offline")
     }
   }
 
+  fun currentCanvasHostUrl(): String? = canvasHostUrl
+
   suspend fun sendEvent(event: String, payloadJson: String?) {
     val conn = currentConnection ?: return
     conn.sendJson(
@@ -209,6 +213,7 @@ class BridgeSession(
         when (first["type"].asStringOrNull()) {
           "hello-ok" -> {
             val name = first["serverName"].asStringOrNull() ?: "Bridge"
+            canvasHostUrl = first["canvasHostUrl"].asStringOrNull()?.trim()?.ifEmpty { null }
             onConnected(name, conn.remoteAddress)
           }
           "error" -> {

apps/android/app/src/test/java/com/steipete/clawdis/node/bridge/BridgeSessionTest.kt

@@ -46,7 +46,7 @@ class BridgeSessionTest {
 
             val hello = reader.readLine()
             assertTrue(hello.contains("\"type\":\"hello\""))
-            writer.write("""{"type":"hello-ok","serverName":"Test Bridge"}""")
+            writer.write("""{"type":"hello-ok","serverName":"Test Bridge","canvasHostUrl":"http://127.0.0.1:18793"}""")
             writer.write("\n")
             writer.flush()
 
@@ -77,6 +77,7 @@ class BridgeSessionTest {
     )
 
     connected.await()
+    assertEquals("http://127.0.0.1:18793", session.currentCanvasHostUrl())
     val payload = session.request(method = "health", paramsJson = null)
     assertEquals("""{"value":123}""", payload)
     server.await()