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.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "new-york",
+  "rsc": true,
+  "tsx": true,
+  "tailwind": {
+    "config": "tailwind.config.ts",
+    "css": "src/app/globals.css",
+    "baseColor": "neutral",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "iconLibrary": "lucide",
+  "rtl": false,
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  },
+  "registries": {}
+}