refactor(frontend): restructure project layout and update API schema

Relocate frontend source code from `next-app/` to `frontend/` to align with the new project structure. This includes removing the old Next.js boilerplate files and establishing a cleaner workspace.

Additionally, updates the OpenAPI specification to include support for the `immich` widget type and its corresponding configuration schema.

- Move frontend files to `frontend/`
- Delete obsolete `next-app/` directory and its configuration
- Add `immich` widget type to `openapi.yaml`
- Update `FrontendPlan.md` with dashboard refactor and UX direction

frontend/components/ui/alert.tsx

@@ -0,0 +1,40 @@
+import * as React from "react";
+import { cva, type VariantProps } from "class-variance-authority";
+import { cn } from "@/lib/utils";
+
+const alertVariants = cva(
+  "relative w-full rounded-lg border px-4 py-3 text-sm [&>svg+div]:translate-y-[-3px] [&>svg]:absolute [&>svg]:left-4 [&>svg]:top-3 [&>svg]:text-foreground [&>svg~*]:pl-7",
+  {
+    variants: {
+      variant: {
+        default: "bg-background text-foreground",
+        destructive: "border-destructive/50 text-destructive dark:border-destructive [&>svg]:text-destructive",
+      },
+    },
+    defaultVariants: { variant: "default" },
+  },
+);
+
+const Alert = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement> & VariantProps<typeof alertVariants>
+>(({ className, variant, ...props }, ref) => (
+  <div ref={ref} role="alert" className={cn(alertVariants({ variant }), className)} {...props} />
+));
+Alert.displayName = "Alert";
+
+const AlertTitle = React.forwardRef<HTMLParagraphElement, React.HTMLAttributes<HTMLHeadingElement>>(
+  ({ className, ...props }, ref) => (
+    <h5 ref={ref} className={cn("mb-1 font-medium leading-none tracking-tight", className)} {...props} />
+  ),
+);
+AlertTitle.displayName = "AlertTitle";
+
+const AlertDescription = React.forwardRef<HTMLParagraphElement, React.HTMLAttributes<HTMLParagraphElement>>(
+  ({ className, ...props }, ref) => (
+    <div ref={ref} className={cn("text-sm [&_p]:leading-relaxed", className)} {...props} />
+  ),
+);
+AlertDescription.displayName = "AlertDescription";
+
+export { Alert, AlertTitle, AlertDescription };