Add shared UI primitives, contexts, and reusable components

Add shadcn components: avatar, breadcrumb, checkbox, collapsible,
command, dialog, dropdown-menu, label, popover, scroll-area, sheet,
skeleton, tabs, textarea, tooltip. Add shared components: BreadcrumbBar,
CommandPalette, CompanySwitcher, CommentThread, EmptyState, EntityRow,
FilterBar, InlineEditor, MetricCard, PageSkeleton, PriorityIcon,
PropertiesPanel, StatusIcon, SidebarNavItem/Section. Add contexts for
breadcrumbs, dialogs, and side panels. Add keyboard shortcut hook and
utility helpers. Update layout, sidebar, and main app shell.

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

ui/src/lib/groupBy.ts

@@ -0,0 +1,11 @@
+export function groupBy<T>(items: T[], keyFn: (item: T) => string): Record<string, T[]> {
+  const result: Record<string, T[]> = {};
+  for (const item of items) {
+    const key = keyFn(item);
+    if (!result[key]) {
+      result[key] = [];
+    }
+    result[key].push(item);
+  }
+  return result;
+}