feat(errors): provide readable Redmine API error messages

- Introduce RedmineAPIError with human-friendly messages.
- Distinguish AuthError, ForbiddenError, HTTP status codes, timeouts
  and connection errors in client.py.
- Update CLI to print the readable message instead of generic
  "Redmine API error: ...".
- Log original exception with traceback when --debug is enabled.
- Add tests for all error paths and CLI output.

Closes #39

redmine_reporter/cli.py

@@ -7,7 +7,7 @@ from datetime import datetime
 from typing import List, Optional
 
 from . import __version__
-from .client import fetch_issues_with_spent_time
+from .client import RedmineAPIError, fetch_issues_with_spent_time
 from .config import Config
 from .formatters.factory import get_console_formatter, get_formatter_by_extension
 from .report_builder import build_grouped_report, calculate_summary
@@ -106,8 +106,13 @@ def main(argv: Optional[List[str]] = None) -> int:
 
     try:
         issue_hours = fetch_issues_with_spent_time(from_date, to_date)
+    except RedmineAPIError as e:
+        print(f"❌ {e.message}", file=sys.stderr)
+        if args.debug and e.original is not None:
+            logging.exception("Original Redmine API error")
+        return 1
     except Exception as e:
-        print(f"❌ Redmine API error: {e}", file=sys.stderr)
+        print(f"❌ Unexpected error: {e}", file=sys.stderr)
         return 1
 
     if issue_hours is None: