feat: Show error toast when concurrency limit is reached

When a user tries to drag a card from backlog to in_progress while at the
max concurrency limit, a toast notification now appears explaining why the
action was blocked and suggesting solutions.

- Added sonner toast library
- Show error toast with dynamic message based on max concurrency
- Updated test utilities with toast helpers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/package.json

@@ -32,6 +32,7 @@
     "next": "16.0.7",
     "react": "19.2.0",
     "react-dom": "19.2.0",
+    "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "zustand": "^5.0.9"
   },