Add delete confirmation dialog for kanban cards

When clicking the trash icon to delete a feature card, users now see a
confirmation dialog asking them to confirm the deletion. This prevents
accidental deletions and provides a clear cancel option.

- Added Dialog component with confirm/cancel buttons to kanban-card
- Updated mock electron API to support test feature injection via __mockFeatures
- Added test utilities for delete confirmation dialog interactions
- Fixed test infrastructure to properly load mock features

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

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

app/src/components/ui/checkbox.tsx

@@ -0,0 +1,30 @@
+"use client";
+
+import * as React from "react";
+import * as CheckboxPrimitive from "@radix-ui/react-checkbox";
+import { Check } from "lucide-react";
+
+import { cn } from "@/lib/utils";
+
+const Checkbox = React.forwardRef<
+  React.ElementRef<typeof CheckboxPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof CheckboxPrimitive.Root>
+>(({ className, ...props }, ref) => (
+  <CheckboxPrimitive.Root
+    ref={ref}
+    className={cn(
+      "peer h-4 w-4 shrink-0 rounded-sm border border-primary ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50 data-[state=checked]:bg-primary data-[state=checked]:text-primary-foreground",
+      className
+    )}
+    {...props}
+  >
+    <CheckboxPrimitive.Indicator
+      className={cn("flex items-center justify-center text-current")}
+    >
+      <Check className="h-4 w-4" />
+    </CheckboxPrimitive.Indicator>
+  </CheckboxPrimitive.Root>
+));
+Checkbox.displayName = CheckboxPrimitive.Root.displayName;
+
+export { Checkbox };