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-lock.json

@@ -25,6 +25,7 @@
         "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"
       },
@@ -11150,6 +11151,16 @@
         "node": ">= 6.0.0"
       }
     },
+    "node_modules/sonner": {
+      "version": "2.0.7",
+      "resolved": "https://registry.npmjs.org/sonner/-/sonner-2.0.7.tgz",
+      "integrity": "sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc",
+        "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
+      }
+    },
     "node_modules/source-map": {
       "version": "0.6.1",
       "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.6.1.tgz",