refactor(canvas): host A2UI via gateway

apps/ios/Sources/Bridge/BridgeSession.swift

@@ -25,6 +25,11 @@ actor BridgeSession {
     private var serverEventSubscribers: [UUID: AsyncStream<BridgeEventFrame>.Continuation] = [:]
 
     private(set) var state: State = .idle
+    private var canvasHostUrl: String?
+
+    func currentCanvasHostUrl() -> String? {
+        self.canvasHostUrl
+    }
 
     func currentRemoteAddress() -> String? {
         guard let endpoint = self.connection?.currentPath?.remoteEndpoint else { return nil }
@@ -101,6 +106,7 @@ actor BridgeSession {
         if base.type == "hello-ok" {
             let ok = try self.decoder.decode(BridgeHelloOk.self, from: data)
             self.state = .connected(serverName: ok.serverName)
+            self.canvasHostUrl = ok.canvasHostUrl?.trimmingCharacters(in: .whitespacesAndNewlines)
             await onConnected?(ok.serverName)
         } else if base.type == "error" {
             let err = try self.decoder.decode(BridgeErrorFrame.self, from: data)
@@ -210,6 +216,7 @@ actor BridgeSession {
         self.connection = nil
         self.queue = nil
         self.buffer = Data()
+        self.canvasHostUrl = nil
 
         let pending = self.pendingRPC.values
         self.pendingRPC.removeAll()

apps/ios/Sources/Model/NodeAppModel.swift

@@ -143,6 +143,13 @@ final class NodeAppModel {
         }
     }
 
+    private func resolveA2UIHostURL() async -> String? {
+        guard let raw = await self.bridge.currentCanvasHostUrl() else { return nil }
+        let trimmed = raw.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty, let base = URL(string: trimmed) else { return nil }
+        return base.appendingPathComponent("__clawdis__/a2ui/").absoluteString
+    }
+
     func setScenePhase(_ phase: ScenePhase) {
         switch phase {
         case .background:
@@ -436,12 +443,22 @@ final class NodeAppModel {
                 return BridgeInvokeResponse(id: req.id, ok: true, payloadJSON: payload)
 
             case ClawdisCanvasA2UICommand.reset.rawValue:
-                self.screen.showDefaultCanvas()
+                guard let a2uiUrl = await self.resolveA2UIHostURL() else {
+                    return BridgeInvokeResponse(
+                        id: req.id,
+                        ok: false,
+                        error: ClawdisNodeError(
+                            code: .unavailable,
+                            message: "A2UI_HOST_NOT_CONFIGURED: gateway did not advertise canvas host"))
+                }
+                self.screen.navigate(to: a2uiUrl)
                 if await !self.screen.waitForA2UIReady(timeoutMs: 5000) {
                     return BridgeInvokeResponse(
                         id: req.id,
                         ok: false,
-                        error: ClawdisNodeError(code: .unavailable, message: "A2UI not ready"))
+                        error: ClawdisNodeError(
+                            code: .unavailable,
+                            message: "A2UI_HOST_UNAVAILABLE: A2UI host not reachable"))
                 }
 
                 let json = try await self.screen.eval(javaScript: """
@@ -468,12 +485,22 @@ final class NodeAppModel {
                     }
                 }
 
-                self.screen.showDefaultCanvas()
+                guard let a2uiUrl = await self.resolveA2UIHostURL() else {
+                    return BridgeInvokeResponse(
+                        id: req.id,
+                        ok: false,
+                        error: ClawdisNodeError(
+                            code: .unavailable,
+                            message: "A2UI_HOST_NOT_CONFIGURED: gateway did not advertise canvas host"))
+                }
+                self.screen.navigate(to: a2uiUrl)
                 if await !self.screen.waitForA2UIReady(timeoutMs: 5000) {
                     return BridgeInvokeResponse(
                         id: req.id,
                         ok: false,
-                        error: ClawdisNodeError(code: .unavailable, message: "A2UI not ready"))
+                        error: ClawdisNodeError(
+                            code: .unavailable,
+                            message: "A2UI_HOST_UNAVAILABLE: A2UI host not reachable"))
                 }
 
                 let messagesJSON = try ClawdisCanvasA2UIJSONL.encodeMessagesJSONArray(messages)

apps/ios/Sources/Screen/ScreenController.swift

@@ -199,11 +199,6 @@ final class ScreenController {
         name: "scaffold",
         ext: "html",
         subdirectory: "CanvasScaffold")
-    private static let a2uiIndexURL: URL? = ScreenController.bundledResourceURL(
-        name: "index",
-        ext: "html",
-        subdirectory: "CanvasA2UI")
-
     func isTrustedCanvasUIURL(_ url: URL) -> Bool {
         guard url.isFileURL else { return false }
         let std = url.standardizedFileURL
@@ -212,11 +207,6 @@ final class ScreenController {
         {
             return true
         }
-        if let expected = Self.a2uiIndexURL,
-           std == expected.standardizedFileURL
-        {
-            return true
-        }
         return false
     }