Simplify auth, remove abstraction layers, add shadcn components

- Delete lib/api/errors.ts and lib/api/auth-guard.ts, inline
  NextResponse.json() error responses directly in route handlers
- Replace non-standard amber CSS variables with primary/primary-foreground
- Regenerate shadcn UI components with biome-ignore comments
- Add CLAUDE.md for repository guidance
- Update dependencies and pnpm lockfile

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components/ui/alert.tsx

@@ -0,0 +1,76 @@
+import { cva, type VariantProps } from "class-variance-authority";
+import type * as React from "react";
+
+import { cn } from "@/lib/utils";
+
+const alertVariants = cva(
+  "grid gap-0.5 rounded-lg border px-2 py-1.5 text-left text-xs/relaxed has-data-[slot=alert-action]:relative has-data-[slot=alert-action]:pr-18 has-[>svg]:grid-cols-[auto_1fr] has-[>svg]:gap-x-1.5 *:[svg]:row-span-2 *:[svg]:translate-y-0.5 *:[svg]:text-current *:[svg:not([class*='size-'])]:size-3.5 w-full relative group/alert",
+  {
+    variants: {
+      variant: {
+        default: "bg-card text-card-foreground",
+        destructive:
+          "text-destructive bg-card *:data-[slot=alert-description]:text-destructive/90 *:[svg]:text-current",
+      },
+    },
+    defaultVariants: {
+      variant: "default",
+    },
+  },
+);
+
+function Alert({
+  className,
+  variant,
+  ...props
+}: React.ComponentProps<"div"> & VariantProps<typeof alertVariants>) {
+  return (
+    <div
+      data-slot="alert"
+      role="alert"
+      className={cn(alertVariants({ variant }), className)}
+      {...props}
+    />
+  );
+}
+
+function AlertTitle({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="alert-title"
+      className={cn(
+        "[&_a]:hover:text-foreground font-medium group-has-[>svg]/alert:col-start-2 [&_a]:underline [&_a]:underline-offset-3",
+        className,
+      )}
+      {...props}
+    />
+  );
+}
+
+function AlertDescription({
+  className,
+  ...props
+}: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="alert-description"
+      className={cn(
+        "text-muted-foreground [&_a]:hover:text-foreground text-xs/relaxed text-balance md:text-pretty [&_a]:underline [&_a]:underline-offset-3 [&_p:not(:last-child)]:mb-4",
+        className,
+      )}
+      {...props}
+    />
+  );
+}
+
+function AlertAction({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="alert-action"
+      className={cn("absolute top-1.5 right-2", className)}
+      {...props}
+    />
+  );
+}
+
+export { Alert, AlertTitle, AlertDescription, AlertAction };