fix(shell): coordinate startup cover and tab stills

crates/waybetter-host/src/tabs.rs

@@ -74,6 +74,9 @@ pub struct TabState {
     /// The language the site's own UI is in (its `<html lang>`), a validated tag; site tabs only.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub lang: Option<String>,
+    /// Site content reached its first meaningful paint (sent only by participating apps).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub content_ready: Option<bool>,
 }
 
 impl TabState {
@@ -93,6 +96,7 @@ impl TabState {
             loading: None,
             unread: None,
             lang: None,
+            content_ready: None,
         }
     }
 }