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/scroll-area.tsx

@@ -0,0 +1,48 @@
+"use client"
+
+import * as React from "react"
+import * as ScrollAreaPrimitive from "@radix-ui/react-scroll-area"
+
+import { cn } from "@/lib/utils"
+
+const ScrollArea = React.forwardRef<
+  React.ElementRef<typeof ScrollAreaPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof ScrollAreaPrimitive.Root>
+>(({ className, children, ...props }, ref) => (
+  <ScrollAreaPrimitive.Root
+    ref={ref}
+    className={cn("relative overflow-hidden", className)}
+    {...props}
+  >
+    <ScrollAreaPrimitive.Viewport className="h-full w-full rounded-[inherit]">
+      {children}
+    </ScrollAreaPrimitive.Viewport>
+    <ScrollBar />
+    <ScrollAreaPrimitive.Corner />
+  </ScrollAreaPrimitive.Root>
+))
+ScrollArea.displayName = ScrollAreaPrimitive.Root.displayName
+
+const ScrollBar = React.forwardRef<
+  React.ElementRef<typeof ScrollAreaPrimitive.ScrollAreaScrollbar>,
+  React.ComponentPropsWithoutRef<typeof ScrollAreaPrimitive.ScrollAreaScrollbar>
+>(({ className, orientation = "vertical", ...props }, ref) => (
+  <ScrollAreaPrimitive.ScrollAreaScrollbar
+    ref={ref}
+    orientation={orientation}
+    className={cn(
+      "flex touch-none select-none transition-colors",
+      orientation === "vertical" &&
+        "h-full w-2.5 border-l border-l-transparent p-[1px]",
+      orientation === "horizontal" &&
+        "h-2.5 flex-col border-t border-t-transparent p-[1px]",
+      className
+    )}
+    {...props}
+  >
+    <ScrollAreaPrimitive.ScrollAreaThumb className="relative flex-1 rounded-full bg-border" />
+  </ScrollAreaPrimitive.ScrollAreaScrollbar>
+))
+ScrollBar.displayName = ScrollAreaPrimitive.ScrollAreaScrollbar.displayName
+
+export { ScrollArea, ScrollBar }