feat: Phase 4-5 — demo mode, polish, deploy, and bug fixes

Add demo mode with mock data provider, Docker deployment, Playwright
tests, PostHog analytics, error boundaries, and SEO metadata. Fix
residences API response unwrapping, kanban drag-and-drop with optimistic
updates, trailing slash proxy redirects, and column name mismatches with
Go API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,34 @@
+# Build stage
+FROM node:20-alpine AS builder
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# Production stage
+FROM node:20-alpine AS production
+
+WORKDIR /app
+
+# Create non-root user
+RUN addgroup -g 1001 nodejs && adduser -u 1001 -G nodejs -s /bin/sh -D nextjs
+
+# Copy standalone build
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+COPY --from=builder --chown=nextjs:nodejs /app/public ./public
+
+USER nextjs
+
+EXPOSE 3000
+ENV PORT=3000
+ENV HOSTNAME="0.0.0.0"
+
+HEALTHCHECK --interval=30s --timeout=10s --start-period=10s --retries=3 \
+    CMD curl -f http://localhost:3000/ || exit 1
+
+CMD ["node", "server.js"]