feat: support user preview configuration and document UI

docs/ui.md

@@ -0,0 +1,39 @@
+# Latchd user interface
+
+Latchd starts on a virtual terminal. The interface keeps the session list and the login fields visible at the same time.
+
+![Login screen](images/latchd-preview.png)
+
+The screenshot comes from the Docker test image. It uses preview mode. Preview mode does not contact PAM or start a user session.
+
+## Help panel
+
+Press `?` to show the key reference.
+
+![Keyboard help](images/latchd-help.png)
+
+Use the following keys:
+
+- Press `Tab` or `Shift-Tab` to move focus.
+- Press the arrow keys to choose a session.
+- Press `/` to filter sessions or accounts.
+- Press `Ctrl-N` or `Ctrl-P` to change the account.
+- Press `Ctrl-R` to refresh account and session discovery.
+- Press `Ctrl-B` to toggle the background.
+- Press `Ctrl-M` to toggle reduced motion.
+- Press `Enter` to authenticate.
+- Press `Esc` to clear the password or close the help panel.
+- Press `Ctrl-C` to restore the terminal and quit.
+
+These messages show account state before login. PAM accepts or rejects the login.
+
+## Capture method
+
+The capture uses a Docker PTY and the release binary from `latchd-test:local`:
+
+```sh
+tmux new-session -d -x 120 -y 36 \
+  'docker run --rm -it latchd-test:local /workspace/target/release/latchd --preview --no-log'
+```
+
+The image in this directory contains the plain terminal capture. It does not show a host VT, a host compositor, or a real PAM login.