fix: make start builds images automatically

docker compose up --build ensures the API and webui images are always built
from the current source on first install or after code changes, so users
don't need a separate make build-api step.

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

Makefile

@@ -56,7 +56,7 @@ init-peers:
 # Management commands
 start:
 	@echo "Starting Personal Internet Cell..."
-	$(DC) up -d
+	$(DC) up -d --build
 	@echo "Services started. Check status with 'make status'"
 
 stop: