feat(cli): add natural language date parsing via dateparser

- Add _parse_natural() to utils.py using dateparser as a fallback when
  structured date formats (YYYY-MM-DD, MM-DD, DD-MM) don't match
- Supports expressions like 'today', 'yesterday', 'monday', '3 days ago'
- Change day argument to nargs='*' and join tokens so unquoted
  multi-word expressions like: uv run timesheets 3 days ago work correctly
- Pin dateparser to English to avoid locale-dependent behaviour
- Update tests to cover natural language cases and fix test_last_monday
  (dateparser does not support 'last monday'; use 'monday' instead)

tests/test_utils.py

@@ -166,9 +166,39 @@ class TestParseDateArg:
         with pytest.raises(ValueError):
             self._parse("13-32")
 
-    def test_unrecognised_format(self):
-        with pytest.raises(ValueError, match="Unrecognised"):
-            parse_date_arg("not-a-date")
+    def test_unrecognised_format_falls_back_to_natural(self):
+        # A value that looks nothing like a date should raise via _parse_natural
+        with pytest.raises(ValueError):
+            parse_date_arg("not-a-date-at-all-xyz")
 
     def test_whitespace_stripped(self):
         assert parse_date_arg("  2026-05-22  ") == date(2026, 5, 22)
+
+
+class TestParseDateArgNatural:
+    """Tests for natural language fallback via dateparser."""
+
+    def test_today(self):
+        from datetime import date as date_cls
+
+        result = parse_date_arg("today")
+        assert result == date_cls.today()
+
+    def test_yesterday(self):
+        from datetime import date as date_cls
+        from datetime import timedelta
+
+        result = parse_date_arg("yesterday")
+        assert result == date_cls.today() - timedelta(days=1)
+
+    def test_weekday_name(self):
+        from datetime import date as date_cls
+
+        result = parse_date_arg("monday")
+        # Must be a Monday and not in the future
+        assert result.weekday() == 0
+        assert result <= date_cls.today()
+
+    def test_garbage_raises(self):
+        with pytest.raises(ValueError):
+            parse_date_arg("not-a-date-at-all-xyz")