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/page_view_page.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module PageViewPage
+  def self.configure(tui)
+    tui.page :page_view do
+      render do
+        text @page_meta[0, cont_w], x: CONT_X, y: CONT_Y, style: THEME[:normal]
+        (@detail[@scroll, cont_h - 2] || []).each_with_index do |l, i|
+          text l[0, cont_w], x: CONT_X, y: CONT_Y + 2 + i, style: THEME[:normal]
+        end
+        text "#{@scroll + 1}/#{@detail.size}  ↑↓ scroll  q back",
+             x: CONT_X, y: CONT_Y + cont_h, style: THEME[:normal]
+      end
+
+      nav :scroll, content: -> { @detail }, window: -> { cont_h - 2 }
+      key('q')     { @scroll = 0; go(@prev_page || :idle) }
+      key(:escape) { @scroll = 0; go(@prev_page || :idle) }
+    end
+  end
+end