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/cli.py

@@ -51,12 +51,12 @@ def build_parser() -> argparse.ArgumentParser:
     )
     parser.add_argument(
         "day",
-        nargs="?",
+        nargs="*",
         help=(
-            "Day to extract timesheets for. Accepts YYYY-MM-DD, MM-DD, or DD-MM "
-            "(- or / as separator). Defaults to today."
+            "Day to extract timesheets for. Accepts YYYY-MM-DD, MM-DD, DD-MM, "
+            "or a natural language expression like 'yesterday' or '3 days ago'. "
+            "Defaults to today."
         ),
-        default=None,
     )
     parser.add_argument(
         "--map",
@@ -93,11 +93,13 @@ def main() -> None:
     config_path = args.config if args.config is not None else find_default_config()
     config = load_config(config_path)
 
-    if args.day is None:
+    day_input = " ".join(args.day) if args.day else None
+
+    if day_input is None:
         target_date = date.today()
     else:
         try:
-            target_date = parse_date_arg(args.day)
+            target_date = parse_date_arg(day_input)
         except AmbiguousDateError as e:
             print(f"Error: {e}", file=sys.stderr)
             sys.exit(1)