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

tests/test_config.py

@@ -99,6 +99,25 @@ def test_get_default_date_range_fallback():
     assert result == f"{start.isoformat()}--{today.isoformat()}"
 
 
+@mock.patch.dict(os.environ, {}, clear=True)
+def test_get_default_date_range_datetime_dynamic_sliding_window():
+    app = AppConfig(
+        period_precision="datetime",
+        period_dynamic=True,
+        period_last_used_to="2026-09-10T15:00:00+00:00",
+    )
+    Config._app = app
+    try:
+        real_dt = __import__("datetime").datetime
+        with mock.patch("redmine_reporter.config.datetime") as dt:
+            dt.fromisoformat = real_dt.fromisoformat
+            dt.now.return_value = real_dt(2026, 9, 15, 10, 0, 0)
+            rng = Config.get_default_date_range()
+    finally:
+        Config._app = None
+    assert rng == "2026-09-10T15:00:00--2026-09-15T10:00:00"
+
+
 # -- #56: дефолтный период — текущий месяц (детерминированные тесты) --