Replace subprocess calls to wg genkey/pubkey with cryptography
library's X25519PrivateKey. This eliminates the wg CLI dependency
for key generation, fixes device creation on machines without
wireguard-tools, and removes the event loop blocking that caused
WebSocket disconnects during device creation.
Also fix E2E test teardown to use a fresh engine for cleanup,
avoiding cross-event-loop issues with asyncpg connection pools.
The ORM-based cleanup couldn't see devices created by the NiceGUI
app's session, so the user delete hit a FK constraint. Raw SQL
DELETE in correct order (children first) works reliably.
10 E2E tests using NiceGUI's User fixture:
- Device creation flow and name validation
- Password change (success, wrong current, mismatch, too short)
- API token creation, TOTP registration, invalid code rejection
- Account deletion with email confirmation
Tests live in tests/e2e/ with a separate conftest that loads the
NiceGUI testing plugin. CI runs unit and E2E tests as separate steps.
