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/client.go

@@ -184,16 +184,30 @@ func (c *AgentClient) authenticate() error {
 	return nil
 }
 
+func jobPayloadHasError(payload json.RawMessage) bool {
+	_, ok := jobPayloadErrorMessage(payload)
+	return ok
+}
+
+func jobPayloadErrorMessage(payload json.RawMessage) (string, bool) {
+	var raw map[string]json.RawMessage
+	if err := json.Unmarshal(payload, &raw); err != nil {
+		return "", false
+	}
+	errMsg, ok := raw["error"]
+	if !ok {
+		return "", false
+	}
+	return strings.Trim(string(errMsg), `"`), true
+}
+
 func (c *AgentClient) handleMessage(msg Message) {
 	switch msg.Type {
 	case "new_job":
-		var raw map[string]json.RawMessage
-		if err := json.Unmarshal(msg.Payload, &raw); err != nil {
-			log.Printf("[agent] bad job payload: %v", err)
-			return
-		}
-		if errMsg, ok := raw["error"]; ok {
-			log.Printf("[agent] job error from server: %s", string(errMsg))
+		if jobPayloadHasError(msg.Payload) {
+			if msg, _ := jobPayloadErrorMessage(msg.Payload); msg != "" {
+				log.Printf("[agent] job error from server: %s", msg)
+			}
 			c.write(Message{Type: "get_job", Payload: json.RawMessage("{}")})
 			return
 		}