Add concurrency slider for automode to control max parallel agents

- Added maxConcurrency state to app-store with persistence
- Created slider UI component using Radix UI
- Added concurrency slider to board-view header (left of Auto Mode button)
- Updated use-auto-mode hook to expose canStartNewTask based on limit
- Block dragging features to in_progress when at max concurrency
- Added test utilities for concurrency slider interactions

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

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

app/package.json

@@ -22,6 +22,7 @@
     "@radix-ui/react-dialog": "^1.1.15",
     "@radix-ui/react-dropdown-menu": "^2.1.16",
     "@radix-ui/react-label": "^2.1.8",
+    "@radix-ui/react-slider": "^1.3.6",
     "@radix-ui/react-slot": "^1.2.4",
     "@radix-ui/react-tabs": "^1.1.13",
     "@tanstack/react-query": "^5.90.12",