Add date_format config key for csv command

- Add get_date_format() to config.py: reads [csv] date_format, returns
  None when absent (falls back to the default %d/%m/%y)
- _cmd_csv applies the format to both single-day and weekly date strings
- Add 3 tests for get_date_format in TestGetters
- Update timesheets.example.toml and README with [csv] date_format key

timesheets.example.toml

@@ -7,3 +7,6 @@ map = "/path/to/project_map.json"
 [work]
 daily_hours = 8.0
 weekly_hours = 40.0
+
+[csv]
+# date_format = "%Y-%m-%d"   # default is "%d/%m/%y" (e.g. 28/05/26)