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.
Implement a mobile-friendly navigation system that replaces the static nav menu with a hamburger toggle button on screens ≤768px. The menu slides in from the left as a fixed overlay panel with smooth CSS transitions, and automatically closes when any nav link is clicked. Includes animated hamburger icon transformation (rotating top/bottom bars, hiding middle bar) and adjusted hero section padding for better mobile layout.
- 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`
