fix: use snack.Manager interface type for capability assertions in integration tests

Type assertions require an interface type, not a concrete struct pointer.
Changed all integration tests from 'mgr := pkg.New()' to
'var mgr snack.Manager = pkg.New()' so capability checks like
mgr.(snack.VersionQuerier) compile correctly.

flatpak/flatpak_integration_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 func TestIntegration_Flatpak(t *testing.T) {
-	mgr := flatpak.New()
+	var mgr snack.Manager = flatpak.New()
 	if !mgr.Available() {
 		t.Skip("flatpak not available — install it first")
 	}