Add full test suite with unit, integration, and E2E smoke tests.

Introduce test.bat orchestrating Go/Vitest/Playwright phases, expand coverage across server, agent, and dashboard, and document in tests/README.md.

agent/client/jobs_test.go

@@ -0,0 +1,31 @@
+package client
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestJobPayloadDetectsServerError(t *testing.T) {
+	raw := json.RawMessage(`{"error":"pool not connected"}`)
+	if !jobPayloadHasError(raw) {
+		t.Fatal("expected error detection")
+	}
+	msg, ok := jobPayloadErrorMessage(raw)
+	if !ok || msg != "pool not connected" {
+		t.Fatalf("unexpected message: %q ok=%v", msg, ok)
+	}
+}
+
+func TestJobPayloadNoErrorOnValidJob(t *testing.T) {
+	raw := json.RawMessage(`{"job_id":"abc","blob":"deadbeef","height":1}`)
+	if jobPayloadHasError(raw) {
+		t.Fatal("valid job should not report error")
+	}
+}
+
+func TestJobPayloadEmptyBlobNotErrorField(t *testing.T) {
+	raw := json.RawMessage(`{"job_id":"abc","blob":""}`)
+	if jobPayloadHasError(raw) {
+		t.Fatal("empty blob is not an error field")
+	}
+}