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/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,