refactor(ui): extract critical severity into a constant

The critical-event gate compared against a bare "critical" literal while
the matching string is produced from the proto SystemEvent_CRITICAL enum in
systemEventFromProto. Add services.SeverityCritical next to the Status*
constants so the two sides share one definition.

client/ui/services/daemon_feed.go

@@ -46,6 +46,10 @@ const (
 	StatusNeedsLogin     = "NeedsLogin"
 	StatusLoginFailed    = "LoginFailed"
 	StatusSessionExpired = "SessionExpired"
+
+	// SeverityCritical is the lower-cased proto SystemEvent_CRITICAL severity, as
+	// emitted by systemEventFromProto. Critical events bypass the notifications gate.
+	SeverityCritical = "critical"
 )
 
 // Emitter sends a named payload to the frontend. Satisfied by Wails app.Event.

client/ui/tray_events.go

@@ -53,7 +53,7 @@ func (t *Tray) onSystemEvent(ev *application.CustomEvent) {
 		return
 	}
 
-	critical := strings.EqualFold(se.Severity, "critical")
+	critical := strings.EqualFold(se.Severity, services.SeverityCritical)
 	t.profileMu.Lock()
 	enabled := t.notificationsEnabled
 	t.profileMu.Unlock()