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/src/components/ui/slider.tsx

@@ -0,0 +1,27 @@
+"use client";
+
+import * as React from "react";
+import * as SliderPrimitive from "@radix-ui/react-slider";
+import { cn } from "@/lib/utils";
+
+const Slider = React.forwardRef<
+  React.ComponentRef<typeof SliderPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof SliderPrimitive.Root>
+>(({ className, ...props }, ref) => (
+  <SliderPrimitive.Root
+    ref={ref}
+    className={cn(
+      "relative flex w-full touch-none select-none items-center",
+      className
+    )}
+    {...props}
+  >
+    <SliderPrimitive.Track className="relative h-1.5 w-full grow overflow-hidden rounded-full bg-white/10">
+      <SliderPrimitive.Range className="absolute h-full bg-gradient-to-r from-purple-600 to-blue-600" />
+    </SliderPrimitive.Track>
+    <SliderPrimitive.Thumb className="block h-4 w-4 rounded-full border border-white/20 bg-zinc-800 shadow transition-colors focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-white/50 disabled:pointer-events-none disabled:opacity-50 hover:bg-zinc-700" />
+  </SliderPrimitive.Root>
+));
+Slider.displayName = SliderPrimitive.Root.displayName;
+
+export { Slider };