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)

src/timesheets/utils.py

@@ -63,10 +63,29 @@ def parse_date_arg(value: str) -> date:
             raise ValueError(f"Invalid date: {value!r}")
         return result
 
-    raise ValueError(
-        f"Unrecognised date format: {value!r}. "
-        "Expected YYYY-MM-DD, MM-DD, or DD-MM (- or / as separator)."
+    # Nothing matched structured formats — try natural language
+    return _parse_natural(value)
+
+
+def _parse_natural(value: str) -> date:
+    """
+    Parse a natural language date string using dateparser.
+    Raises ValueError if the string cannot be interpreted.
+    """
+    import dateparser
+
+    result = dateparser.parse(
+        value,
+        languages=["en"],
+        settings={"PREFER_DATES_FROM": "past", "RETURN_AS_TIMEZONE_AWARE": False},
     )
+    if result is None:
+        raise ValueError(
+            f"Could not interpret {value!r} as a date. "
+            "Try a structured format like '2026-05-22' or '05-22', "
+            "or a relative expression like 'today', 'yesterday', 'monday', or '3 days ago'."
+        )
+    return result.date()
 
 
 def parse_duration(duration_str: str) -> float: