Enhance v2 TUI with connected/mls_epoch state fields, colored connection
indicator in status bar, MLS epoch display, and wildcard match for new
SDK event variants. Add 9 tests using ratatui TestBackend covering
rendering, navigation, scroll bounds, status bar content, and unread
count display. Also fix rand 0.8 compat issue in v2_repl.rs.
Add v2_main.rs and v2_commands.rs as thin wrappers over QpqClient from
quicproquo-sdk. Activated via --features v2 (feature-gated so v1 main.rs
remains the default). Includes auto-server-launch, clap-based command
surface (register-user, login, whoami, health, resolve, dm, group,
devices), and ClientConfig-based connection setup. MLS-dependent commands
(send, recv, group create/invite) print stubs pointing to the REPL.