shell: read the instance file synchronously, or it is empty when asked

FileView loads async, so text() was still empty when onReadyChanged read it
once to decide whether this is a new session. It fell through to the unset
env var and persisted "" — the same bug, reached by a different route.
Verified on device: the stored signature now matches viewtop.instance.

surfaces/quickshell/modules/common/Persistent.qml

@@ -46,6 +46,12 @@ Singleton {
         // Absence is normal, not an error: a Hyprland session has no such file
         // and falls through to the env var below.
         printErrors: false
+        // Synchronously, because `onReadyChanged` reads `text()` exactly once
+        // to decide whether this is a new session. Loaded async it was still
+        // empty at that moment, fell through to the unset env var, and the
+        // signature persisted as "" — the same bug this replaced, arrived at
+        // by a different route.
+        blockLoading: true
     }
 
     onReadyChanged: {