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/Dockerfile

@@ -0,0 +1,24 @@
+FROM node:22-alpine AS base
+WORKDIR /app
+COPY package.json package-lock.json* ./
+RUN npm ci
+
+FROM base AS builder
+WORKDIR /app
+COPY --from=base /app/node_modules ./node_modules
+COPY . .
+RUN npm run build
+
+FROM base AS runner
+WORKDIR /app
+ENV NODE_ENV=production
+RUN addgroup --system --gid 1001 nodejs
+RUN adduser --system --uid 1001 nextjs
+COPY --from=builder /app/public ./public
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+USER nextjs
+EXPOSE 3000
+ENV PORT=3000
+ENV HOSTNAME="0.0.0.0"
+CMD ["node", "server.js"]