fix: sliding period window for datetime precision

After --commit with precision=datetime, last_used held from==to==now,
so the next default period degenerated to a single second and showed
no new time entries. Now --commit stores the real report window start
and get_default_date_range() returns last_used.to..now for
datetime+dynamic mode.

Closes #70

redmine_reporter/config.py

@@ -2,7 +2,7 @@ import logging
 import os
 import sys
 from dataclasses import dataclass, field
-from datetime import date, timedelta
+from datetime import date, datetime, timedelta, timezone
 from pathlib import Path
 from typing import Dict, Union
 
@@ -440,6 +440,23 @@ class Config:
         if from_env:
             return f"{from_env}--{to_env or today_str}"
 
+        if (
+            cls._app
+            and cls._app.period_dynamic
+            and (cls._app.period_precision or "date") == "datetime"
+            and cls._app.period_last_used_to
+        ):
+            last_to = datetime.fromisoformat(
+                cls._app.period_last_used_to.replace("Z", "+00:00")
+            )
+            if last_to.tzinfo is None:
+                last_to = last_to.replace(tzinfo=timezone.utc)
+            else:
+                last_to = last_to.astimezone(timezone.utc)
+            now_utc = datetime.now(timezone.utc)
+            fmt = "%Y-%m-%dT%H:%M:%S"
+            return f"{last_to.strftime(fmt)}--{now_utc.strftime(fmt)}"
+
         if (
             cls._app
             and cls._app.period_dynamic