Add a new client-side Path Mapping Helper page at src/app/path-helper/page.tsx. Implements a multi-step wizard to help users configure Docker volume mappings for download clients and ReadMeABook (RMAB): select clients, enter container save paths, enter host/container volume mappings (with optional remote path mapping), and generate recommended RMAB docker-compose volume snippet. Includes utility functions to compute common roots and relative paths, UI components (step indicator, info/warning boxes, code block), and logic to derive RMAB download directory, per-client custom paths, and verification instructions. No API calls — purely client-side helper with sensible defaults for supported clients.