Make the focus exception list real, or it was a page telling a lie

The mode data model shipped with an allow list and nothing that read it. The
summary would say "2 apps may interrupt" while notification delivery never
consulted the list and no editor could set it. That is the dead row this work
has spent its time removing, introduced by the work itself.

The banner gate consults the mode in force now, and the list can be edited from
the applications that have actually sent a notification -- an exception for
something that never notifies is not a choice worth offering.

Exceptions belong to a mode. allowedApps is empty whenever no mode is active, so
a Do Not Disturb switched on by hand stays absolute and nothing can leak into
it. That scoping is asserted, not just written.

Verifying this took three attempts, and the second was a real defect in the
guard rather than in the code. The contract grep for FocusModes.allows matched
the comment that explains it, so the check passed with the enforcement deleted.
It matches the gate expression now. A guard a comment can satisfy is not a
guard, and this is the third time prose has satisfied one here.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/services/Notifs.qml

@@ -234,7 +234,10 @@ Singleton {
                 root.unreadCount += 1;
             }
 
-            if (!root.doNotDisturb) {
+            // An exception belongs to the focus mode that is in force. A Do Not
+            // Disturb switched on by hand has no exceptions and stays absolute,
+            // because FocusModes.allows is false whenever no mode is active.
+            if (!root.doNotDisturb || FocusModes.allows(root.notificationAppId(notification))) {
                 root.popups = [notification].concat(root.popups);
             } else if (notification.transient) {
                 // Never shown, and (being transient) never filed in history