feat: add checksum-based update detection

Makefile

@@ -64,8 +64,10 @@ release: lint
 	@xattr -cr "build/bin/Daily Timer.app" 2>/dev/null || true
 	@rm -rf dist && mkdir -p dist
 	cd build/bin && zip -r "../../dist/Daily-Timer-$(VERSION)-macos-arm64.zip" "Daily Timer.app"
+	@shasum -a 256 "build/bin/Daily Timer.app/Contents/MacOS/daily-timer" | awk '{print $$1}' > "dist/Daily-Timer-$(VERSION)-macos-arm64.sha256"
 	@echo "Release package: dist/Daily-Timer-$(VERSION)-macos-arm64.zip"
-	@ls -lh dist/*.zip
+	@echo "Checksum: $$(cat dist/Daily-Timer-$(VERSION)-macos-arm64.sha256)"
+	@ls -lh dist/*
 
 # Release for both architectures
 release-all: lint
@@ -89,7 +91,7 @@ release-upload:
 		-d '{"tag_name": "$(VERSION)", "name": "$(VERSION)", "body": "Release $(VERSION)"}' \
 		| jq -r '.id'); \
 	echo "Created release ID: $$RELEASE_ID"; \
-	for file in dist/*.zip; do \
+	for file in dist/*; do \
 		filename=$$(basename "$$file"); \
 		echo "Uploading $$filename..."; \
 		curl -s -X POST \