feat: add multi-architecture Docker build setup

- Add Docker BuildKit builder setup for AMD64 and ARM64 platforms
- Update backend and frontend Dockerfiles with platform flags
- Create comprehensive build scripts for multi-arch workflows
- Add verification script to test multi-architecture images
- Update Makefile with multi-arch Docker targets
- Add detailed documentation for multi-architecture setup

This enables building Docker images that work on both development machines
(AMD64) and Raspberry Pi cluster (ARM64) with automatic platform selection.

🤖 Generated with [AI Assistant]

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

frontend/Dockerfile

@@ -1,5 +1,5 @@
-# Multi-stage build for ARM compatibility (Raspberry Pi)
-FROM oven/bun:1-alpine AS builder
+# Multi-stage build for multi-architecture support (AMD64, ARM64)
+FROM --platform=$BUILDPLATFORM oven/bun:1-alpine AS builder
 
 WORKDIR /app
 
@@ -16,7 +16,7 @@ COPY . .
 RUN bun run build
 
 # --- Production stage with nginx ---
-FROM nginx:alpine
+FROM --platform=$TARGETPLATFORM nginx:alpine
 
 # Install wget for health checks
 RUN apk add --no-cache wget