Correct daypart identity and display handling

internal/config/config_test.go

@@ -82,6 +82,40 @@ func TestDefaults(t *testing.T) {
 	}
 }
 
+func TestDaypartNamesMustHaveDistinctCanonicalIdentities(t *testing.T) {
+	tests := []struct {
+		name    string
+		names   []string
+		wantErr string
+	}{
+		{name: "exact duplicate", names: []string{"morning", "morning"}, wantErr: "conflicts with"},
+		{name: "case-only duplicate", names: []string{"morning", "MORNING"}, wantErr: "conflicts with"},
+		{name: "punctuation-normalized duplicate", names: []string{"morning", "morning!"}, wantErr: "conflicts with"},
+		{name: "distinct Unicode names", names: []string{"mañana", "manana"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := Defaults()
+			cfg.Dayparts = make([]DaypartConfig, 0, len(tt.names))
+			for _, name := range tt.names {
+				cfg.Dayparts = append(cfg.Dayparts, DaypartConfig{Name: name, Start: "06:00", End: "12:00"})
+			}
+
+			err := Validate(cfg)
+			if tt.wantErr == "" {
+				if err != nil {
+					t.Fatalf("Validate() error = %v", err)
+				}
+				return
+			}
+			if err == nil || !strings.Contains(err.Error(), tt.wantErr) {
+				t.Fatalf("Validate() error = %v, want %q", err, tt.wantErr)
+			}
+		})
+	}
+}
+
 func TestWeatherAPIBaseURLValidation(t *testing.T) {
 	tests := []struct {
 		name    string