feat: dashboard error boundaries, loading guards, and data import settings page

- Add page-level and layout-level error.tsx boundaries for dashboard
- Add loading guard to dashboard page to defer API calls until auth is ready
- Fix Dockerfile to accept NEXT_PUBLIC_API_URL as build arg
- Fix .env.example API URL to include /api suffix
- Add Data Import tab in settings for CSV/Excel import of clients and invoices
- Create import page with template download, drag-drop upload, and row-level results

Dockerfile

@@ -5,6 +5,8 @@ COPY packages/shared/package.json ./packages/shared/
 RUN npm install
 COPY packages/shared/ ./packages/shared/
 COPY . .
+ARG NEXT_PUBLIC_API_URL=http://localhost:3001/api
+ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
 RUN npx next build
 
 FROM node:20-alpine AS runner