Extract grouping logic into shared function (#19)

Add group_rows_by_project_and_version() to report_builder.py and replace
the duplicated inline grouping in both html.py and odt.py with a single
call to the shared function. This ensures consistent grouping behavior
across formatters and removes a maintenance burden — any change to
grouping logic now happens in one place.

Closes #19

redmine_reporter/formatters/html.py

@@ -1,6 +1,7 @@
 from html import escape
-from typing import Dict, List
+from typing import List
 
+from ..report_builder import group_rows_by_project_and_version
 from ..types import ReportRow
 from .base import Formatter
 
@@ -12,16 +13,7 @@ class HTMLFormatter(Formatter):
         super().__init__()
 
     def format(self, rows: List[ReportRow]) -> str:
-        # Сгруппируем данные
-        projects: Dict[str, Dict[str, List[ReportRow]]] = {}
-        for r in rows:
-            proj = r["project"]
-            ver = r["version"]
-            if proj not in projects:
-                projects[proj] = {}
-            if ver not in projects[proj]:
-                projects[proj][ver] = []
-            projects[proj][ver].append(r)
+        projects = group_rows_by_project_and_version(rows)
 
         lines = [
             '<table border="1" cellpadding="6" cellspacing="0" style="border-collapse: collapse; font-family: Arial, sans-serif;">',