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

redmine_reporter/formatters/html.py

@@ -48,7 +48,7 @@ class HTMLFormatter(Formatter):
                 for r in task_rows:
                     task_cell = escape(f"{r['issue_id']}. {r['subject']}")
                     status_text = escape(r["status_ru"])
-                    time_text = escape(r["time_text"])
+                    time_text = escape(r["time_text"]).replace("\n", "<br>")
                     lines.append("    <tr>")
 
                     # Ячейка "Проект" - только в первой строке проекта