fix: prevent install buttons showing before first container scan

Added containers_scanned flag to StatusInfo in the data model. Starts
false, set to true after the first Podman scan completes (~15s after
boot). Marketplace now shows a shimmer "Checking..." indicator on app
buttons until the scan finishes, preventing users from accidentally
re-installing apps that are already present but not yet enumerated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/data_model.rs

@@ -66,6 +66,10 @@ pub struct StatusInfo {
     pub backup_progress: Option<f32>,
     #[serde(rename = "update-progress")]
     pub update_progress: Option<f32>,
+    /// True after the first container scan completes. Frontend should
+    /// not show install buttons until this is true.
+    #[serde(rename = "containers-scanned", default)]
+    pub containers_scanned: bool,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -253,6 +257,7 @@ impl DataModel {
                     updated: false,
                     backup_progress: None,
                     update_progress: None,
+                    containers_scanned: false,
                 },
                 lan_address: Some("http://localhost:8100".to_string()),
                 tor_address: None,