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

README.md

@@ -108,6 +108,7 @@ Categorize your work sessions (e.g., Remote, Onsite, Meeting). The current work
 | `/edit YYYY-MM-DD`          | View/edit events for a specific date                        |
 | `/note [DATE] HH:MM <text>` | Set note for an event at a specific time                    |
 | `/settimezone <IANA>`       | Set timezone (e.g. `Asia/Tehran`)                           |
+| `/setcalendar <type>`       | Set calendar type (`gregorian`, `jalali`, `hijri`)          |
 | `/setbreak <minutes>`       | Set minimum break threshold (0-480, default 5)              |
 | `/setaccent`                | Select export accent color                                  |
 | `/setreporttime HH:MM`      | Set daily auto-report time                                  |