feat(coach): swap the sources-rail toggle to a shadcn Checkbox

Replace the raw <input type="checkbox"> in the sources rail with a
shadcn-style <Checkbox> backed by radix-ui. The new primitive ships
a real focus ring, a Space-toggles keyboard contract, and a touch-
friendly hit target so the rail behaves correctly on the right-edge
side-tray surface mobile users open via the chevron button.

src/components/ui/checkbox.tsx

@@ -0,0 +1,47 @@
+"use client";
+
+import * as React from "react";
+import { Checkbox as CheckboxPrimitive } from "radix-ui";
+import { Check } from "lucide-react";
+
+import { cn } from "@/lib/utils";
+
+/**
+ * v1.4.27 R3d MB4 / CF-40 — shadcn-style `<Checkbox>` primitive backed
+ * by `radix-ui`'s `Checkbox` namespace. Brought in so the Coach
+ * sources-rail can drop its raw `<input type="checkbox">` for a
+ * keyboard + touch accessible control that ships proper focus rings
+ * and a 44 px hit target on mobile.
+ *
+ * Mirrors the v1.4.27 shadcn `<Checkbox>` recipe: a Root that draws
+ * the border + checked-state background, an Indicator slot that
+ * shows the lucide `<Check>` when the box is checked.
+ */
+function Checkbox({
+  className,
+  ...props
+}: React.ComponentProps<typeof CheckboxPrimitive.Root>) {
+  return (
+    <CheckboxPrimitive.Root
+      data-slot="checkbox"
+      className={cn(
+        "peer border-border/70 size-4 shrink-0 rounded border",
+        "data-[state=checked]:bg-dracula-purple data-[state=checked]:border-dracula-purple data-[state=checked]:text-background",
+        "focus-visible:ring-ring/50 focus-visible:ring-2 focus-visible:outline-none",
+        "disabled:cursor-not-allowed disabled:opacity-50",
+        "transition-colors",
+        className,
+      )}
+      {...props}
+    >
+      <CheckboxPrimitive.Indicator
+        data-slot="checkbox-indicator"
+        className="flex items-center justify-center text-current"
+      >
+        <Check className="size-3" aria-hidden="true" />
+      </CheckboxPrimitive.Indicator>
+    </CheckboxPrimitive.Root>
+  );
+}
+
+export { Checkbox };