Fix extension context detection in runtime providers

Replace window.sendRuntimeMessage existence check with URL-based detection.
The sendRuntimeMessage function exists in both extension and non-extension
contexts (e.g., downloaded HTML artifacts), causing incorrect behavior.

Changes:
- Add window.__isExtensionContext() helper to RuntimeMessageBridge that checks:
  - chrome-extension:// URLs (Chrome)
  - moz-extension:// URLs (Firefox)
  - about:srcdoc (sandbox iframes)
- Update all runtime providers to use __isExtensionContext() instead:
  - FileDownloadRuntimeProvider: Correctly falls back to browser download
  - ConsoleRuntimeProvider: Only sends messages in extension context
  - ArtifactsRuntimeProvider: Properly detects offline/read-only mode

This fixes the issue where downloaded HTML artifacts incorrectly try to
communicate with the extension when opened from disk.

packages/web-ui/src/components/sandbox/ArtifactsRuntimeProvider.ts

@@ -43,7 +43,7 @@ export class ArtifactsRuntimeProvider implements SandboxRuntimeProvider {
 
 			(window as any).listArtifacts = async (): Promise<string[]> => {
 				// Online: ask extension
-				if ((window as any).sendRuntimeMessage) {
+				if ((window as any).__isExtensionContext?.()) {
 					const response = await (window as any).sendRuntimeMessage({
 						type: "artifact-operation",
 						action: "list",
@@ -61,7 +61,7 @@ export class ArtifactsRuntimeProvider implements SandboxRuntimeProvider {
 				let content: string;
 
 				// Online: ask extension
-				if ((window as any).sendRuntimeMessage) {
+				if ((window as any).__isExtensionContext?.()) {
 					const response = await (window as any).sendRuntimeMessage({
 						type: "artifact-operation",
 						action: "get",
@@ -94,7 +94,7 @@ export class ArtifactsRuntimeProvider implements SandboxRuntimeProvider {
 				content: any,
 				mimeType?: string,
 			): Promise<void> => {
-				if (!(window as any).sendRuntimeMessage) {
+				if (!(window as any).__isExtensionContext?.()) {
 					throw new Error("Cannot create/update artifacts in offline mode (read-only)");
 				}
 
@@ -117,7 +117,7 @@ export class ArtifactsRuntimeProvider implements SandboxRuntimeProvider {
 			};
 
 			(window as any).deleteArtifact = async (filename: string): Promise<void> => {
-				if (!(window as any).sendRuntimeMessage) {
+				if (!(window as any).__isExtensionContext?.()) {
 					throw new Error("Cannot delete artifacts in offline mode (read-only)");
 				}
 

packages/web-ui/src/components/sandbox/ConsoleRuntimeProvider.ts

@@ -51,8 +51,8 @@ export class ConsoleRuntimeProvider implements SandboxRuntimeProvider {
 					// Always log locally too
 					(originalConsole as any)[method].apply(console, args);
 
-					// Send immediately and track the promise
-					if ((window as any).sendRuntimeMessage) {
+					// Send immediately and track the promise (only in extension context)
+					if ((window as any).__isExtensionContext?.()) {
 						const sendPromise = (window as any)
 							.sendRuntimeMessage({
 								type: "console",
@@ -108,7 +108,7 @@ export class ConsoleRuntimeProvider implements SandboxRuntimeProvider {
 
 				const finalError = error || lastError;
 
-				if ((window as any).sendRuntimeMessage) {
+				if ((window as any).__isExtensionContext?.()) {
 					if (finalError) {
 						await (window as any).sendRuntimeMessage({
 							type: "execution-error",

packages/web-ui/src/components/sandbox/FileDownloadRuntimeProvider.ts

@@ -52,8 +52,8 @@ export class FileDownloadRuntimeProvider implements SandboxRuntimeProvider {
 					finalMimeType = mimeType || "application/json";
 				}
 
-				// Send to extension if available (online mode)
-				if ((window as any).sendRuntimeMessage) {
+				// Send to extension if in extension context (online mode)
+				if ((window as any).__isExtensionContext?.()) {
 					const response = await (window as any).sendRuntimeMessage({
 						type: "file-returned",
 						fileName,

packages/web-ui/src/components/sandbox/RuntimeMessageBridge.ts

@@ -28,6 +28,13 @@ export class RuntimeMessageBridge {
 		// Returns stringified function that uses window.parent.postMessage
 		return `
 window.__completionCallbacks = [];
+// Check if we're in an extension context by examining the URL
+window.__isExtensionContext = () => {
+    const url = window.location.href;
+    return url.startsWith('chrome-extension://') ||
+           url.startsWith('moz-extension://') ||
+           url === 'about:srcdoc';
+};
 window.sendRuntimeMessage = async (message) => {
     const messageId = 'msg_' + Date.now() + '_' + Math.random().toString(36).substring(2, 9);
 
@@ -68,6 +75,13 @@ window.onCompleted = (callback) => {
 		// Returns stringified function that uses chrome.runtime.sendMessage
 		return `
 window.__completionCallbacks = [];
+// Check if we're in an extension context by examining the URL
+window.__isExtensionContext = () => {
+    const url = window.location.href;
+    return url.startsWith('chrome-extension://') ||
+           url.startsWith('moz-extension://') ||
+           url === 'about:srcdoc';
+};
 window.sendRuntimeMessage = async (message) => {
     return await chrome.runtime.sendMessage({
         ...message,