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.

docker/Dockerfile.backend

@@ -12,14 +12,16 @@ RUN apk add --no-cache ca-certificates git
 
 WORKDIR /workspace
 
-COPY go.mod go.sum ./
-RUN go mod download
+COPY backend/go.mod backend/go.sum ./backend/
+RUN cd backend && go mod download
 
 COPY . .
 
+WORKDIR /workspace/backend
+
 RUN CGO_ENABLED=0 GOOS=linux go build \
     -trimpath \
-    -ldflags="-s -w -X github.com/Rain-kl/Wavelet/pkg/buildinfo.Version=${VERSION} -X github.com/Rain-kl/Wavelet/pkg/buildinfo.BuildTime=${BUILD_DATE}" \
+    -ldflags="-s -w -X Wavelet/pkg/buildinfo.Version=${VERSION} -X Wavelet/pkg/buildinfo.BuildTime=${BUILD_DATE}" \
     -o /out/wavelet \
     ./main.go