Redesign TUI with DOS Navigator look and two-panel layout

Full blue background (screen_fill), double-line bordered panels side
by side: left panel for the menu, right panel for content. The right
panel's title bar shows the current page name, username and online count.

Color theme (THEME constant) maps to combined fg+bg ANSI codes so all
elements look native on the blue background: bright cyan borders, white
normal text, yellow labels, black-on-cyan selection highlights.

Status bar at the bottom mimics the NC function key bar with bright key
labels on cyan background.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Gemfile.lock

@@ -1,6 +1,6 @@
 GIT
   remote: https://git.teletype.hu/tools/rubbs.git
-  revision: cac10176a25eb5d917f559d4dfb6c7d9c0dc34da
+  revision: 148fd886600c814e3bdb13ec3b3fb3f812d99cb9
   specs:
     bbs (0.2.0)
       artii (~> 2.1)