build: use WEB_GO for web targets and preserve backend dist directory (#1671)

Separate web Go commands from the default Go toolchain so web builds,
tests, and vet can enable CGO on Darwin without affecting the rest of
the project. Also ensure frontend backend builds recreate backend/dist
with a .gitkeep file so the embedded output directory remains tracked.

Makefile

@@ -16,6 +16,7 @@ LDFLAGS=-X $(CONFIG_PKG).Version=$(VERSION) -X $(CONFIG_PKG).GitCommit=$(GIT_COM
 
 # Go variables
 GO?=CGO_ENABLED=0 go
+WEB_GO?=$(GO)
 GOFLAGS?=-v -tags stdjson
 
 # Patch MIPS LE ELF e_flags (offset 36) for NaN2008-only kernels (e.g. Ingenic X2600).
@@ -79,6 +80,7 @@ ifeq ($(UNAME_S),Linux)
 	endif
 else ifeq ($(UNAME_S),Darwin)
 	PLATFORM=darwin
+	WEB_GO=CGO_ENABLED=1 go
 	ifeq ($(UNAME_M),x86_64)
 		ARCH=amd64
 	else ifeq ($(UNAME_M),arm64)
@@ -119,7 +121,7 @@ build-launcher:
 		echo "Building frontend..."; \
 		cd web/frontend && pnpm install && pnpm build:backend; \
 	fi
-	@$(GO) build $(GOFLAGS) -o $(BUILD_DIR)/picoclaw-launcher-$(PLATFORM)-$(ARCH) ./web/backend
+	@$(WEB_GO) build $(GOFLAGS) -o $(BUILD_DIR)/picoclaw-launcher-$(PLATFORM)-$(ARCH) ./web/backend
 	@ln -sf picoclaw-launcher-$(PLATFORM)-$(ARCH) $(BUILD_DIR)/picoclaw-launcher
 	@echo "Build complete: $(BUILD_DIR)/picoclaw-launcher"
 
@@ -219,7 +221,9 @@ clean:
 
 ## vet: Run go vet for static analysis
 vet: generate
-	@$(GO) vet ./...
+	@packages="$$(go list ./...)" && \
+		$(GO) vet $$(printf '%s\n' "$$packages" | grep -v '^github.com/sipeed/picoclaw/web/')
+	@cd web/backend && $(WEB_GO) vet ./...
 
 ## test: Test Go code
 test: generate