feat: add /setcalendar command, store tests, and improve error handling

- Add /setcalendar command for calendar type parity (Settings UI)
- Add 17 store tests covering all repository methods
- Add handler tests for formatDuration, fmtHHMM, fmtDDHHMM
- Add domain tests for BurnoutLevel.String, NavMonth, FormatSalary
- Fix silent error discarding in save* helpers (log write failures)
- Fix computeAndAwardXP error handling instead of discarding
- Wrap critical store.go SQL errors with fmt.Errorf context
- Update README commands table with /setcalendar

internal/handler/handler_test.go

@@ -0,0 +1,70 @@
+package handler
+
+import (
+	"testing"
+)
+
+func TestFormatDuration(t *testing.T) {
+	tests := []struct {
+		seconds int64
+		want    string
+	}{
+		{0, "0m"},
+		{30, "0m"},
+		{59, "0m"},
+		{60, "1m"},
+		{3599, "59m"},
+		{3600, "1h 0m"},
+		{3661, "1h 1m"},
+		{7200, "2h 0m"},
+		{7260, "2h 1m"},
+		{86399, "23h 59m"},
+		{86400, "24h 0m"},
+	}
+	for _, tc := range tests {
+		got := formatDuration(tc.seconds)
+		if got != tc.want {
+			t.Errorf("formatDuration(%d) = %q, want %q", tc.seconds, got, tc.want)
+		}
+	}
+}
+
+func TestFmtHHMM(t *testing.T) {
+	tests := []struct {
+		seconds int64
+		want    string
+	}{
+		{0, "0:00"},
+		{60, "0:01"},
+		{3600, "1:00"},
+		{3661, "1:01"},
+		{86399, "23:59"},
+		{86400, "24:00"},
+	}
+	for _, tc := range tests {
+		got := fmtHHMM(tc.seconds)
+		if got != tc.want {
+			t.Errorf("fmtHHMM(%d) = %q, want %q", tc.seconds, got, tc.want)
+		}
+	}
+}
+
+func TestFmtDDHHMM(t *testing.T) {
+	tests := []struct {
+		seconds int64
+		want    string
+	}{
+		{0, "0:00:00"},
+		{60, "0:00:01"},
+		{3600, "0:01:00"},
+		{86400, "1:00:00"},
+		{90061, "1:01:01"},
+		{172800, "2:00:00"},
+	}
+	for _, tc := range tests {
+		got := fmtDDHHMM(tc.seconds)
+		if got != tc.want {
+			t.Errorf("fmtDDHHMM(%d) = %q, want %q", tc.seconds, got, tc.want)
+		}
+	}
+}