[PDGGK]

Follow-up to #134 (comment), where I said I would add npm run test:ci and go test to the build job.

The build job's frontend step is named "Build and test frontend" but only runs npm run build, and no step runs the Go tests. So the three Jest suites in src/ and the tests in pkg/plugin never run in CI.

This change:

• runs npm run test:ci before npm run build
• adds a Test backend step with go test ./pkg/... after backend-compile.sh. go.sum is not checked in, and the script's go mod tidy is what produces it, so the tests have to come after it.

Checked locally in the same order (Node 22, Go 1.26):

• npm ci then the Jest run: 3 suites, 38 tests pass
• go get -u github.com/grafana/grafana-plugin-sdk-go@v0.250.0 && go mod tidy (the dependency part of backend-compile.sh), then go test ./pkg/...: pkg/plugin passes
• with one assertion broken in tableQuery.test.ts, the Jest run exits 1; with a t.Fatal added to table_query_test.go, go test exits 1