fix(build): add -tags embed to ensure frontend is embedded

- Add -tags=embed flag to GoReleaser builds
- Add -tags embed flag to Dockerfile builds
- Fix Dockerfile COPY order to prevent frontend dist being overwritten
- Update README build instructions with embed tag explanation

.goreleaser.yaml

@@ -11,6 +11,8 @@ builds:
     dir: backend
     main: ./cmd/server
     binary: sub2api
+    flags:
+      - -tags=embed
     env:
       - CGO_ENABLED=0
     goos: