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/context/PanelContext.tsx

@@ -0,0 +1,35 @@
+import { createContext, useCallback, useContext, useState, type ReactNode } from "react";
+
+interface PanelContextValue {
+  panelContent: ReactNode | null;
+  openPanel: (content: ReactNode) => void;
+  closePanel: () => void;
+}
+
+const PanelContext = createContext<PanelContextValue | null>(null);
+
+export function PanelProvider({ children }: { children: ReactNode }) {
+  const [panelContent, setPanelContent] = useState<ReactNode | null>(null);
+
+  const openPanel = useCallback((content: ReactNode) => {
+    setPanelContent(content);
+  }, []);
+
+  const closePanel = useCallback(() => {
+    setPanelContent(null);
+  }, []);
+
+  return (
+    <PanelContext.Provider value={{ panelContent, openPanel, closePanel }}>
+      {children}
+    </PanelContext.Provider>
+  );
+}
+
+export function usePanel() {
+  const ctx = useContext(PanelContext);
+  if (!ctx) {
+    throw new Error("usePanel must be used within PanelProvider");
+  }
+  return ctx;
+}