fix: handle 204 No Content in fetchJSON, add FK-aware delete errors, v2 screenshots

Frontend: fetchJSON now returns empty object on 204 instead of failing
to parse empty body — fixes silent delete failures across all entities.
Added onError callbacks to owner/team delete mutations to surface errors.

Backend: owner and issuer delete handlers return 409 Conflict with
descriptive messages when FK constraints block deletion, instead of
generic 500.

Added 15 v2 dashboard screenshots, updated README screenshot section,
logo asset, page count references (18→full), and QA guide with FK
constraint test coverage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/handler/issuers.go

@@ -184,7 +184,13 @@ func (h IssuerHandler) DeleteIssuer(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if err := h.svc.DeleteIssuer(id); err != nil {
-		ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to delete issuer", requestID)
+		if strings.Contains(err.Error(), "violates foreign key") || strings.Contains(err.Error(), "RESTRICT") {
+			ErrorWithRequestID(w, http.StatusConflict, "Cannot delete issuer: certificates are still using this issuer", requestID)
+		} else if strings.Contains(err.Error(), "not found") {
+			ErrorWithRequestID(w, http.StatusNotFound, "Issuer not found", requestID)
+		} else {
+			ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to delete issuer", requestID)
+		}
 		return
 	}
 

internal/api/handler/owners.go

@@ -183,7 +183,13 @@ func (h OwnerHandler) DeleteOwner(w http.ResponseWriter, r *http.Request) {
 	id = parts[0]
 
 	if err := h.svc.DeleteOwner(id); err != nil {
-		ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to delete owner", requestID)
+		if strings.Contains(err.Error(), "violates foreign key") || strings.Contains(err.Error(), "RESTRICT") {
+			ErrorWithRequestID(w, http.StatusConflict, "Cannot delete owner: certificates are still assigned to this owner", requestID)
+		} else if strings.Contains(err.Error(), "not found") {
+			ErrorWithRequestID(w, http.StatusNotFound, "Owner not found", requestID)
+		} else {
+			ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to delete owner", requestID)
+		}
 		return
 	}