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.
- Update `generate_legal_documents` default `template_dir` from `"templates/legal"` to `"mywebdav/templates/legal"`
- Update `Jinja2Templates` directory from `"templates"` to `"mywebdav/templates"`
- Add new `base.html` template with navigation, footer, and SEO meta tags
- Add new `features.html` template with responsive grid layout and feature cards
- Add new legal document templates: `compliance_statement.html`, `contact_complaint_mechanism.html`, `cookie_policy.html`, `data_portability_deletion_policy.html`
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.
