Add page-based TUI DSL

Each screen is now a self-contained `page` block with its own `enter`,
`render`, and `key` handlers. Shared logic lives in a `helpers` block
mixed into the render context. A `chrome` block renders persistent UI
(header, sidebar, footer) before every frame.

New context primitives: `go(page)` for transitions, `reload` to re-run
the current page's `enter` block, `printable` key binding for text input.

`FlowRunner#run_tui` now passes the flow context to TUIRunner so username
and other session data are available without reaching into internals.

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

bbs.gemspec

@@ -1,6 +1,6 @@
 Gem::Specification.new do |s|
   s.name        = 'bbs'
-  s.version     = '0.1.0'
+  s.version     = '0.2.0'
   s.summary     = 'Universal telnet BBS server library'
   s.author      = 'Zsolt Tasnadi'
   s.files       = Dir['lib/**/*.rb']