fix(docker): repair embed packaging for backend/ module layout

The Dockerfiles and the release workflow still assumed the pre-refactor tree:
go.mod, go.sum and main.go now live under backend/, so `COPY go.mod go.sum`
failed outright, and the Go module is the bare `Wavelet`, so the buildinfo
ldflags pointed at github.com/Rain-kl/Wavelet and stamped nothing. The
frontend export was also overlaid onto ./plugins/... rather than the
driver_http path that `//go:embed all:dist` actually reads.

Repoint every packaging path at backend/, correct the ldflags module prefix,
and assert dist/index.html in both docker stages, in build-embedded and in the
release workflow, so a silently empty static export can no longer ship an
API-only binary. Also carry pnpm-workspace.yaml into the cached install layer.

Makefile

@@ -27,6 +27,7 @@ build-embedded:
 		pnpm build:embed
 	rm -rf backend/plugins/drivers/driver_http/dist
 	cp -R frontend/out backend/plugins/drivers/driver_http/dist
+	test -f backend/plugins/drivers/driver_http/dist/index.html
 	cd backend && go build \
 		-tags embed_frontend \
 		-ldflags "-s -w -X '$(MODULE)/pkg/buildinfo.Version=$(VERSION)' -X '$(MODULE)/pkg/buildinfo.BuildTime=$(BUILD_DATE)'" \