Result box: color-code by status + friendly error messages from error codes (#105)

- Backend create/can validation paths emit machine-readable error codes
  (slug_taken, slug_invalid, content_empty, content_too_large,
  expiry_invalid, rate_limited, ...) alongside the human message
- new.html JS maps codes to plain-language guidance with generic fallback
- Result card colored via --ok/--err/--warn left border (result-ok/err/warn)
- docs/API.md error section documents the code field
- Tests assert the code on every validation path

internal/api/issue105_errorcode_test.go

@@ -0,0 +1,123 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// #105: every validation path on create returns a machine-readable `code`
+// alongside the human `error` message, so the new-page JS can map known
+// codes to plain-language guidance.
+
+func decodeErr(t *testing.T, rec *httptest.ResponseRecorder) (status int, errMsg, code string) {
+	t.Helper()
+	var got struct {
+		Error string `json:"error"`
+		Code  string `json:"code"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
+		t.Fatalf("bad json: %v (%s)", err, rec.Body.String())
+	}
+	return rec.Code, got.Error, got.Code
+}
+
+func postCreate(h http.Handler, body string) *httptest.ResponseRecorder {
+	req := httptest.NewRequest("POST", "/api/pastes", strings.NewReader(body))
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	return rec
+}
+
+func TestErrorCodeContentEmpty(t *testing.T) {
+	h := testServer(t).routes()
+	_, _, code := decodeErr(t, postCreate(h, `{"content":"   "}`))
+	if code != "content_empty" {
+		t.Fatalf("code = %q, want content_empty", code)
+	}
+}
+
+func TestErrorCodeContentTooLarge(t *testing.T) {
+	h := testServer(t).routes()
+	big := strings.Repeat("x", 6*1024*1024) // over the 5MB test cap
+	_, _, code := decodeErr(t, postCreate(h, `{"content":"`+big+`"}`))
+	if code != "content_too_large" {
+		t.Fatalf("code = %q, want content_too_large", code)
+	}
+}
+
+func TestErrorCodeSlugTaken(t *testing.T) {
+	h := testServer(t).routes()
+	if rec := postCreate(h, `{"content":"a","custom_slug":"taken-slug"}`); rec.Code != 201 {
+		t.Fatalf("seed create: %d %s", rec.Code, rec.Body.String())
+	}
+	_, _, code := decodeErr(t, postCreate(h, `{"content":"b","custom_slug":"taken-slug"}`))
+	if code != "slug_taken" {
+		t.Fatalf("code = %q, want slug_taken", code)
+	}
+}
+
+func TestErrorCodeSlugInvalid(t *testing.T) {
+	h := testServer(t).routes()
+	_, _, code := decodeErr(t, postCreate(h, `{"content":"a","custom_slug":"bad slug!"}`))
+	if code != "slug_invalid" {
+		t.Fatalf("code = %q, want slug_invalid", code)
+	}
+}
+
+func TestErrorCodeExpiryInvalid(t *testing.T) {
+	h := testServer(t).routes()
+	_, _, code := decodeErr(t, postCreate(h, `{"content":"a","expires_in":"2s"}`))
+	if code != "expiry_invalid" {
+		t.Fatalf("code = %q, want expiry_invalid", code)
+	}
+}
+
+func TestErrorCodeRateLimited(t *testing.T) {
+	h := testServer(t).routes()
+	var code string
+	for i := 0; i < 20; i++ {
+		_, _, code = decodeErr(t, postCreate(h, `{"content":"a"}`))
+		if code == "rate_limited" {
+			return
+		}
+	}
+	t.Fatalf("never got rate_limited, last code = %q", code)
+}
+
+func TestErrorCodeCanValidation(t *testing.T) {
+	h := testServer(t).routes()
+
+	post := func(fields map[string]string) *httptest.ResponseRecorder {
+		var b strings.Builder
+		for k, v := range fields {
+			b.WriteString("--B\r\nContent-Disposition: form-data; name=\"" + k + "\"\r\n\r\n" + v + "\r\n")
+		}
+		b.WriteString("--B--\r\n")
+		req := httptest.NewRequest("POST", "/api/pastes/can", strings.NewReader(b.String()))
+		req.Header.Set("Content-Type", "multipart/form-data; boundary=B")
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, req)
+		return rec
+	}
+
+	// bad visibility
+	if _, _, code := decodeErr(t, post(map[string]string{"title": "x", "visibility": "nope"})); code != "invalid_visibility" {
+		t.Fatalf("visibility code = %q, want invalid_visibility", code)
+	}
+
+	// invalid expiry
+	if _, _, code := decodeErr(t, post(map[string]string{"title": "x", "expires_in": "1s"})); code != "expiry_invalid" {
+		t.Fatalf("expiry code = %q, want expiry_invalid", code)
+	}
+
+	// slug taken on can create
+	if rec := postCreate(h, `{"content":"a","custom_slug":"can-slug"}`); rec.Code != 201 {
+		t.Fatalf("seed create: %d", rec.Code)
+	}
+	if _, _, code := decodeErr(t, post(map[string]string{"title": "x", "custom_slug": "can-slug", "json_items": `[{"title":"a","content":"b"}]`})); code != "slug_taken" {
+		t.Fatalf("can slug code = %q, want slug_taken", code)
+	}
+}