Extract pages into lib/page/ modules; thin down bbs.rb

Each page is now a self-contained module with a configure(tui) class
method. Shared helpers live in TUIHelpers. bbs.rb is reduced to
constants, chrome, and wiring.

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

lib/page/new_msg_page.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module NewMsgPage
+  def self.configure(tui)
+    tui.page :new_msg do
+      enter { @page_title = 'POST MESSAGE'; @input = +''; @status = +'' }
+
+      render do
+        text 'Type your message and press Enter.  Esc cancels.',
+             x: CONT_X, y: CONT_Y, style: THEME[:normal]
+        text "#{tc(:bright, @username)}: #{tc(:input, @input)}_",
+             x: CONT_X, y: CONT_Y + 2
+        unless @status.empty?
+          text @status, x: CONT_X, y: CONT_Y + 4, style: THEME[:error]
+        end
+      end
+
+      key(:enter) do
+        if @input.strip.empty?
+          @status = 'Message cannot be empty.'
+        else
+          MESSAGES.append(@username, @input.strip[0...200])
+          go :idle
+        end
+      end
+      key(:backspace) { @input.chop! unless @input.empty? }
+      key(:escape)    { go :idle }
+      printable       { |ch| @input << ch if @input.length < 200 }
+    end
+  end
+end