Render GFM tables and enable mouse-wheel scrolling

Markdown.to_lines now detects GFM tables (header + delimiter row) and
renders them as box-drawn, alignment-aware tables that shrink to fit the
pane width, instead of mangling the rows into a single paragraph.

ScrollView now handles mouse-wheel events, and Window routes wheel
events to the widget under the cursor, so a detail/content pane scrolls
even when keyboard focus is on an adjacent list.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/bbs/window.rb

@@ -70,6 +70,16 @@ module BBS
         end
       end
 
+      # Route wheel scrolling to whatever widget is under the cursor, so a
+      # scrollable pane responds even when keyboard focus is elsewhere.
+      if event.mouse? && event.mouse.kind == :wheel
+        hit = hit_test(event.x, event.y)
+        if hit && hit != focus_manager.focused
+          res = hit.handle_event(event)
+          return res if res
+        end
+      end
+
       result = focus_manager.handle_focus_key(event) ? :handled : nil
       result ||= focus_manager.dispatch(event)
       return result if result