feat(ui): complete bb-ui2.2 - shadcn/ui Setup

STORY:
The Unified UX epic needed a solid component foundation. We chose
shadcn/ui for its Tailwind integration and copy-paste philosophy.

COLLABORATION:
Initialized shadcn/ui with Next.js defaults and installed the base
component set needed for the unified shell:
- button: Primary actions
- card: Card containers
- badge: Status badges
- avatar: Agent avatars
- input: Search/filter inputs
- scroll-area: Scrollable containers
- separator: Visual dividers
- tooltip: Hover information
- dropdown-menu: Sorting and filtering

We also updated tsconfig.json with path aliases (@/*) to support
the shadcn import pattern.

DELIVERABLES:
- components.json configuration
- 9 shadcn components in components/ui/
- lib/utils.ts with cn() helper
- tsconfig.json with @/* path aliases

VERIFICATION:
- npm run typecheck: PASS
- npm run lint: PASS

CLOSES: bb-ui2.2
BLOCKS: bb-ui2.3, bb-ui2.5

components/ui/separator.tsx

@@ -0,0 +1,31 @@
+"use client"
+
+import * as React from "react"
+import * as SeparatorPrimitive from "@radix-ui/react-separator"
+
+import { cn } from "@/lib/utils"
+
+const Separator = React.forwardRef<
+  React.ElementRef<typeof SeparatorPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof SeparatorPrimitive.Root>
+>(
+  (
+    { className, orientation = "horizontal", decorative = true, ...props },
+    ref
+  ) => (
+    <SeparatorPrimitive.Root
+      ref={ref}
+      decorative={decorative}
+      orientation={orientation}
+      className={cn(
+        "shrink-0 bg-border",
+        orientation === "horizontal" ? "h-[1px] w-full" : "h-full w-[1px]",
+        className
+      )}
+      {...props}
+    />
+  )
+)
+Separator.displayName = SeparatorPrimitive.Root.displayName
+
+export { Separator }