fix: handle error returns from os.Unsetenv and file.Close (errcheck)

Fixed the first five errcheck linter errors:
- Added error handling for os.Unsetenv in cli_test.go
- Added error handling for file.Close() in crypto.go (4 instances)

internal/cli/cli_test.go

@@ -47,7 +47,7 @@ func TestDetermineStateDir(t *testing.T) {
 	}
 
 	// Test with custom config dir
-	os.Unsetenv(secret.EnvStateDir)
+	_ = os.Unsetenv(secret.EnvStateDir)
 	customConfigDir := "/custom-config"
 	stateDir = secret.DetermineStateDir(customConfigDir)
 	expectedDir := filepath.Join(customConfigDir, secret.AppID)