Tor is being moved from a built-in OS feature to a service. This removes
the Arti-based Tor client, onion address management, hidden service
creation, and all related code from the core backend, frontend, and SDK.
- Delete core/src/net/tor/ module (~2060 lines)
- Remove OnionAddress, TorSecretKey, TorController from all consumers
- Remove HostnameInfo::Onion and HostAddress::Onion variants
- Remove onion CRUD RPC endpoints and tor subcommand
- Remove tor key handling from account and backup/restore
- Remove ~12 tor-related Cargo dependencies (arti-client, torut, etc.)
- Remove tor UI components, API methods, mock data, and routes
- Remove OnionHostname and tor patterns/regexes from SDK
- Add v0_4_0_alpha_20 database migration to strip onion data
- Bump version to 0.4.0-alpha.20
