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

tsconfig.json

@@ -13,7 +13,11 @@
     "isolatedModules": true,
     "jsx": "preserve",
     "incremental": true,
-    "plugins": [{ "name": "next" }]
+    "plugins": [{ "name": "next" }],
+    "baseUrl": ".",
+    "paths": {
+      "@/*": ["./*"]
+    }
   },
   "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx", ".next/types/**/*.ts"],
   "exclude": ["node_modules"]