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/widgets.rb

@@ -459,8 +459,17 @@ module BBS
       end
 
       def handle_event(event)
-        return nil unless event.key?
         max = [@lines.size - bounds.height, 0].max
+        if event.mouse?
+          return nil unless event.mouse.kind == :wheel
+          @scroll = if event.mouse.button == :up
+                      [@scroll - 3, 0].max
+                    else
+                      [@scroll + 3, max].min
+                    end
+          return :handled
+        end
+        return nil unless event.key?
         case event.key
         when :up        then @scroll = [@scroll - 1, 0].max
         when :down      then @scroll = [@scroll + 1, max].min