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/AGENTS.md

@@ -0,0 +1,38 @@
+# Dash Frontend – Agent Rules
+
+## Scope
+- This agent may edit: `/frontend`
+- This agent must not edit `/backend`, `/db`, or `/openapi`
+
+## Tech Stack
+- Next.js 15 App Router + React 19 + TypeScript strict
+- Tailwind CSS v4 + shadcn/ui (new-york style)
+- `@tanstack/react-query` for server state
+- `@dnd-kit` for drag-and-drop
+- `openapi-typescript` + `openapi-fetch` for API client (generated from `../openapi/openapi.yaml`)
+
+## Commands
+- `npm run dev` — start dev server (Turbopack)
+- `npm run build` — production build
+- `npm run typecheck` — `tsc --noEmit`
+- `npm run lint` — Next.js lint
+- `npm run api:generate` — regenerate API types from OpenAPI spec
+
+## Design
+- Dark-first, Vercel-inspired aesthetic
+- 3 themes: light, dark, casaos (glassmorphism)
+- Geist Sans + Geist Mono fonts
+- Shadow-as-border technique (no visible borders, use box-shadow)
+- See `../Design.md` for full design system
+
+## API Contract
+- All types come from `../openapi/openapi.yaml`
+- Do not invent contract fields outside OpenAPI
+- API base URL: `NEXT_PUBLIC_API_BASE_URL` (default `http://localhost:8080`)
+
+## Component Rules
+- Use shadcn/ui primitives, do not rebuild from scratch
+- All interactive elements must have focus rings
+- Prefer `font-mono uppercase tracking-wide` for labels/badges
+- Service cards are square aspect-ratio, icon + name + URL badges
+- Groups are collapsible sections with chevron toggle