refactor: split the TUI and refresh project docs

Move login state, authentication, discovery, input, rendering, and tests into focused TUI modules. Add detailed CLI error reports with recovery commands, align the build and host-check documentation, and keep the legacy Go tree outside normal builds.

Checks: make check; make integration-test; go test ./... (legacy/go).

scripts/test-cli.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Use --preview so this test does not need PAM, a VT, or a display server.
+# Use --preview; this test needs no credentials, VT, or display server.
 set -eu
 
 binary=${1:-target/debug/latchd}