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/hooks/use-auto-mode.ts

@@ -17,6 +17,7 @@ export function useAutoMode() {
     clearRunningTasks,
     currentProject,
     addAutoModeActivity,
+    maxConcurrency,
   } = useAppStore(
     useShallow((state) => ({
       isAutoModeRunning: state.isAutoModeRunning,
@@ -27,9 +28,13 @@ export function useAutoMode() {
       clearRunningTasks: state.clearRunningTasks,
       currentProject: state.currentProject,
       addAutoModeActivity: state.addAutoModeActivity,
+      maxConcurrency: state.maxConcurrency,
     }))
   );
 
+  // Check if we can start a new task based on concurrency limit
+  const canStartNewTask = runningAutoTasks.length < maxConcurrency;
+
   // Handle auto mode events
   useEffect(() => {
     const api = getElectronAPI();
@@ -178,6 +183,8 @@ export function useAutoMode() {
   return {
     isRunning: isAutoModeRunning,
     runningTasks: runningAutoTasks,
+    maxConcurrency,
+    canStartNewTask,
     start,
     stop,
   };