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/sheet.tsx

@@ -0,0 +1,87 @@
+"use client";
+
+import * as React from "react";
+import * as DialogPrimitive from "@radix-ui/react-dialog";
+import { cva, type VariantProps } from "class-variance-authority";
+import { X } from "lucide-react";
+import { cn } from "@/lib/utils";
+
+const Sheet = DialogPrimitive.Root;
+const SheetTrigger = DialogPrimitive.Trigger;
+const SheetClose = DialogPrimitive.Close;
+const SheetPortal = DialogPrimitive.Portal;
+
+const SheetOverlay = React.forwardRef<
+  React.ComponentRef<typeof DialogPrimitive.Overlay>,
+  React.ComponentPropsWithoutRef<typeof DialogPrimitive.Overlay>
+>(({ className, ...props }, ref) => (
+  <DialogPrimitive.Overlay
+    className={cn("fixed inset-0 z-50 bg-black/80 data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0", className)}
+    {...props}
+    ref={ref}
+  />
+));
+SheetOverlay.displayName = DialogPrimitive.Overlay.displayName;
+
+const sheetVariants = cva(
+  "fixed z-50 gap-4 bg-background p-6 shadow-lg transition ease-in-out data-[state=closed]:duration-300 data-[state=open]:duration-500 data-[state=open]:animate-in data-[state=closed]:animate-out",
+  {
+    variants: {
+      side: {
+        top: "inset-x-0 top-0 border-b data-[state=closed]:slide-out-to-top data-[state=open]:slide-in-from-top",
+        bottom: "inset-x-0 bottom-0 border-t data-[state=closed]:slide-out-to-bottom data-[state=open]:slide-in-from-bottom",
+        left: "inset-y-0 left-0 h-full w-3/4 border-r data-[state=closed]:slide-out-to-left data-[state=open]:slide-in-from-left sm:max-w-sm",
+        right: "inset-y-0 right-0 h-full w-3/4 border-l data-[state=closed]:slide-out-to-right data-[state=open]:slide-in-from-right sm:max-w-sm",
+      },
+    },
+    defaultVariants: { side: "right" },
+  },
+);
+
+interface SheetContentProps
+  extends React.ComponentPropsWithoutRef<typeof DialogPrimitive.Content>,
+    VariantProps<typeof sheetVariants> {}
+
+const SheetContent = React.forwardRef<React.ComponentRef<typeof DialogPrimitive.Content>, SheetContentProps>(
+  ({ side = "right", className, children, ...props }, ref) => (
+    <SheetPortal>
+      <SheetOverlay />
+      <DialogPrimitive.Content ref={ref} className={cn(sheetVariants({ side }), className)} {...props}>
+        <DialogPrimitive.Close className="absolute right-4 top-4 rounded-sm opacity-70 ring-offset-background transition-opacity hover:opacity-100 focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2">
+          <X className="h-4 w-4" />
+          <span className="sr-only">Close</span>
+        </DialogPrimitive.Close>
+        {children}
+      </DialogPrimitive.Content>
+    </SheetPortal>
+  ),
+);
+SheetContent.displayName = DialogPrimitive.Content.displayName;
+
+const SheetHeader = ({ className, ...props }: React.HTMLAttributes<HTMLDivElement>) => (
+  <div className={cn("flex flex-col space-y-2 text-center sm:text-left", className)} {...props} />
+);
+SheetHeader.displayName = "SheetHeader";
+
+const SheetFooter = ({ className, ...props }: React.HTMLAttributes<HTMLDivElement>) => (
+  <div className={cn("flex flex-col-reverse sm:flex-row sm:justify-end sm:space-x-2", className)} {...props} />
+);
+SheetFooter.displayName = "SheetFooter";
+
+const SheetTitle = React.forwardRef<
+  React.ComponentRef<typeof DialogPrimitive.Title>,
+  React.ComponentPropsWithoutRef<typeof DialogPrimitive.Title>
+>(({ className, ...props }, ref) => (
+  <DialogPrimitive.Title ref={ref} className={cn("text-lg font-semibold text-foreground", className)} {...props} />
+));
+SheetTitle.displayName = DialogPrimitive.Title.displayName;
+
+const SheetDescription = React.forwardRef<
+  React.ComponentRef<typeof DialogPrimitive.Description>,
+  React.ComponentPropsWithoutRef<typeof DialogPrimitive.Description>
+>(({ className, ...props }, ref) => (
+  <DialogPrimitive.Description ref={ref} className={cn("text-sm text-muted-foreground", className)} {...props} />
+));
+SheetDescription.displayName = DialogPrimitive.Description.displayName;
+
+export { Sheet, SheetTrigger, SheetClose, SheetContent, SheetHeader, SheetFooter, SheetTitle, SheetDescription };