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.

.github/workflows/build-release.yml

@@ -12,6 +12,7 @@ on:
 
 env:
   APP_NAME: wavelet
+  GO_DIR: backend
   GO_MAIN: ./main.go
   GO_BUILD_TAGS: embed_frontend
   GO_LDFLAGS: -s -w
@@ -20,7 +21,7 @@ env:
   FRONTEND_DIR: frontend
   FRONTEND_BUILD_COMMAND: pnpm build:embed
   FRONTEND_OUT_DIR: frontend/out
-  EMBED_DIST_DIR: internal/router/root/dist
+  EMBED_DIST_DIR: backend/plugins/drivers/driver_http/dist
   EXTRA_FILES: |
     LICENSE
     README.md
@@ -192,7 +193,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v5
         with:
-          go-version-file: go.mod
+          go-version-file: ${{ env.GO_DIR }}/go.mod
           cache: true
 
       - name: Build binary
@@ -212,18 +213,21 @@ jobs:
             binary_name="${binary_name}.exe"
           fi
 
-          ldflags="$GO_LDFLAGS -X github.com/Rain-kl/Wavelet/internal/buildinfo.Version=$VERSION -X github.com/Rain-kl/Wavelet/internal/buildinfo.BuildTime=$BUILD_DATE"
+          # Assert the embedded UI is present so a release cannot ship an API-only binary.
+          test -f "$EMBED_DIST_DIR/index.html"
+
+          ldflags="$GO_LDFLAGS -X Wavelet/pkg/buildinfo.Version=$VERSION -X Wavelet/pkg/buildinfo.BuildTime=$BUILD_DATE"
           build_args=(
             -trimpath
             -ldflags "$ldflags"
-            -o "dist/$binary_name"
+            -o "$GITHUB_WORKSPACE/dist/$binary_name"
           )
 
           if [[ -n "$GO_BUILD_TAGS" ]]; then
             build_args=(-tags "$GO_BUILD_TAGS" "${build_args[@]}")
           fi
 
-          go build "${build_args[@]}" "$GO_MAIN"
+          (cd "$GO_DIR" && go build "${build_args[@]}" "$GO_MAIN")
 
       - name: Package artifact
         id: package