feat(kanban): Make In Progress column double-width with 2-column masonry layout

Modified KanbanColumn to accept isDoubleWidth prop.
In Progress column now renders at 37rem width (double the standard 18rem).
Cards in In Progress display in a CSS columns-2 masonry layout.
Other columns maintain their standard single-column width.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/components/layout/sidebar.tsx

@@ -10,7 +10,6 @@ import {
   Settings,
   FileText,
   LayoutGrid,
-  Code,
   Bot,
   ChevronLeft,
   ChevronRight,
@@ -69,7 +68,6 @@ export function Sidebar() {
       items: [
         { id: "spec", label: "Spec Editor", icon: FileText },
         { id: "context", label: "Context", icon: BookOpen },
-        { id: "code", label: "Code View", icon: Code },
         { id: "tools", label: "Agent Tools", icon: Wrench },
       ],
     },