Fix mobile browser tombstoning detection with conservative approach to preserve user state (#90)

* Initial plan

* Fix mobile browser tombstoning detection for blob URLs

Co-authored-by: danmarshall <11507384+danmarshall@users.noreply.github.com>

* Simplify tombstoning detection to avoid sandbox restrictions

- Replace direct document access with conservative blob URL detection
- Add health check message types to common messages (for future use)
- Always recreate blob URL sandboxes on visibility change to ensure mobile tombstoning recovery

Co-authored-by: danmarshall <11507384+danmarshall@users.noreply.github.com>

* Address feedback: revert UMD file, remove half-implemented healthcheck, make restoration less aggressive

Co-authored-by: danmarshall <11507384+danmarshall@users.noreply.github.com>

* Make tombstoning detection less aggressive to preserve user state during tab switching

Co-authored-by: danmarshall <11507384+danmarshall@users.noreply.github.com>

* Revert packages/common/src/messages.ts to remove whitespace changes

Co-authored-by: danmarshall <11507384+danmarshall@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: danmarshall <11507384+danmarshall@users.noreply.github.com>

Author: Copilot

packages/host/src/listener.ts

@@ -288,28 +288,31 @@ export class Listener {
   }
 
   /**
-   * Check if the sandbox iframe is still functional by testing if we can access its content window
+   * Check if the sandbox iframe is still functional
+   * Conservative approach that only detects clear evidence of tombstoning
    */
   private isSandboxFunctional(): boolean {
-    try {
-      if (!this.sandbox || !this.sandbox.iframe) {
-        return false;
-      }
-      
-      // Try to access the iframe's content window and check if the src is still valid
-      const iframe = this.sandbox.iframe;
-      const contentWindow = iframe.contentWindow;
-      
-      // If we can't access the content window or the src is invalid, sandbox is not functional
-      if (!contentWindow || !iframe.src || iframe.src === 'about:blank') {
-        return false;
-      }
-      
-      return true;
-    } catch (error) {
-      // If accessing iframe throws an error, it's not functional
+    if (!this.sandbox || !this.sandbox.iframe) {
+      return false;
+    }
+    
+    const iframe = this.sandbox.iframe;
+    const contentWindow = iframe.contentWindow;
+    
+    // Only recreate if we have clear evidence of a broken iframe
+    // Missing contentWindow is a clear sign of tombstoning
+    if (!contentWindow) {
+      return false;
+    }
+    
+    // Missing or invalid src indicates a problem
+    if (!iframe.src || iframe.src === 'about:blank') {
       return false;
     }
+    
+    // For normal cases (including blob URLs), assume functional to preserve user state
+    // Only the clear failures above will trigger recreation
+    return true;
   }
 
   /**