Feature: portable session export/import with user password

[davidgr4th]

Problem

Sessions are encrypted with a key derived from hostname + username.
This means:

• Can't transfer a session to a CI server or Docker container
• Can't share a session between machines
• Can't back up sessions portably

Proposal

Add export/import commands:

web2cli session export x.com --password "mypass" > x-session.enc
web2cli session import x-session.enc --password "mypass"

Behavior:
- Export encrypts session with user-provided password (not machine-derived key)
- Uses PBKDF2 + Fernet (same as current store, but with real secret)
- Import decrypts and saves to local session store
- Password never stored on disk

Use cases

- Log in once on laptop, deploy session to CI/CD pipeline
- Run web2cli agents in Docker/Kubernetes with pre-authenticated sessions
- Team sharing: one person logs in, exports session for the bot
- Backup/restore when switching machines