Implement a full-featured admin backend at `/manage/` with login authentication using
signed cookies and constant-time credential verification. The panel includes a dashboard
showing total/active users, storage usage, monthly revenue, and pending invoices, plus
user management, payment overview, and pricing configuration pages. New environment
variables `ADMIN_USERNAME`, `ADMIN_PASSWORD`, `ADMIN_SESSION_SECRET`, and
`ADMIN_SESSION_EXPIRE_HOURS` control access. The admin router is registered in main.py
and all new templates, auth module, and router files are added.
Replace every occurrence of "RBox" with "MyWebdav" in source files, configuration,
legal documents, UI components, and test fixtures. This includes renaming the
`RBoxApp` class to `MyWebdavApp` in the JavaScript component, updating the
TOTP issuer from "RBox" to "MyWebdav" in the auth router, changing the
application description in the argument parser, and updating all user-facing
strings in HTML templates, legal policies, and the manifest file. The
`.env.example` default for `TOTP_ISSUER` is also updated accordingly.
