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/odt.py

@@ -125,8 +125,12 @@ class ODTFormatter(Formatter):
                     row.addElement(status_cell)
 
                     time_cell = TableCell(stylename=cell_style_name)
-                    p = P(stylename=para_style_name, text=r["time_text"])
-                    time_cell.addElement(p)
+                    time_lines = r["time_text"].split("\n")
+                    for i, line in enumerate(time_lines):
+                        p = P(stylename=para_style_name, text=line)
+                        time_cell.addElement(p)
+                        if i < len(time_lines) - 1:
+                            time_cell.addElement(P(stylename=para_style_name, text=""))
                     row.addElement(time_cell)
 
                     table.addElement(row)