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/separator.tsx

@@ -0,0 +1,16 @@
+import * as React from "react";
+import { cn } from "@/lib/utils";
+
+const Separator = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDivElement> & { orientation?: "horizontal" | "vertical" }>(
+  ({ className, orientation = "horizontal", ...props }, ref) => (
+    <div
+      ref={ref}
+      role="separator"
+      className={cn("shrink-0 bg-border", orientation === "horizontal" ? "h-[1px] w-full" : "h-full w-[1px]", className)}
+      {...props}
+    />
+  ),
+);
+Separator.displayName = "Separator";
+
+export { Separator };