feat(activity): add --by-activity flag to break down spent time by activity type

- Load time entry activity names from Redmine enumeration.
- Aggregate hours per issue and per activity in fetch_issues_with_spent_time.
- Add --by-activity CLI flag and propagate it through report builder,
  summary, and all formatters (console, CSV, HTML, JSON, ODT).
- Keep backward-compatible 2-tuple input in build_grouped_report.
- Bump version to 1.8.0.

Closes #41

tests/test_client.py

@@ -45,7 +45,7 @@ def test_fetch_aggregates_hours_per_issue(mock_redmine_class):
 
     assert result is not None
     assert len(result) == 1
-    issue, total_hours = result[0]
+    issue, total_hours, _activities = result[0]
     assert total_hours == 3.5
 
 
@@ -109,7 +109,7 @@ def test_fetch_multiple_issues(mock_redmine_class):
     assert result is not None
     assert len(result) == 2
 
-    hours_by_id = {issue.id: hours for issue, hours in result}
+    hours_by_id = {issue.id: hours for issue, hours, _ in result}
     assert hours_by_id[1] == 1.5
     assert hours_by_id[2] == 2.0