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/server.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"database/sql"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"html/template"
 	"net/http"
@@ -51,6 +52,30 @@ func writeErr(w http.ResponseWriter, status int, msg string) {
 	writeJSON(w, status, map[string]string{"error": msg})
 }
 
+// writeErrCode emits a JSON error with a machine-readable code (#105): the
+// new-page JS maps known codes to plain-language messages, API consumers get
+// the stable `code` field alongside the human `error` text.
+func writeErrCode(w http.ResponseWriter, status int, code, msg string) {
+	writeJSON(w, status, map[string]string{"error": msg, "code": code})
+}
+
+// createErrCode maps a store.CreatePaste/CreateCan error to its UI error
+// code (#105).
+func createErrCode(err error) string {
+	switch {
+	case errors.Is(err, store.ErrSlugTaken):
+		return "slug_taken"
+	case errors.Is(err, store.ErrReservedSlug):
+		return "slug_reserved"
+	case errors.Is(err, store.ErrInvalidSlug):
+		return "slug_invalid"
+	case strings.Contains(err.Error(), "expires_in"):
+		return "expiry_invalid"
+	default:
+		return "invalid_request"
+	}
+}
+
 // Routes returns the HTTP handler for the server.
 func (a *apiServer) Routes() http.Handler {
 	return a.routes()
@@ -158,14 +183,18 @@ func (a *apiServer) handleCreatePaste(w http.ResponseWriter, r *http.Request) {
 	var p store.Paste
 	if err := json.NewDecoder(r.Body).Decode(&p); err != nil {
 		if isBodyTooLarge(err) { // #68: body cut off by MaxBytesReader
-			writeBodyTooLarge(w)
+			writeErrCode(w, http.StatusRequestEntityTooLarge, "content_too_large", "request body too large")
 			return
 		}
 		writeErr(w, 400, "invalid json body")
 		return
 	}
 	if status, msg := checkContent(p.Content, s.MaxContentBytes); status != 0 {
-		writeErr(w, status, msg)
+		if status == http.StatusRequestEntityTooLarge {
+			writeErrCode(w, status, "content_too_large", msg)
+		} else {
+			writeErrCode(w, status, "content_empty", msg)
+		}
 		return
 	}
 	// #86: bound free-form metadata at create time
@@ -203,7 +232,7 @@ func (a *apiServer) handleCreatePaste(w http.ResponseWriter, r *http.Request) {
 	p.ViewerID = currentViewerID(r)
 	created, err := a.store.CreatePaste(&p)
 	if err != nil {
-		writeErr(w, 400, err.Error())
+		writeErrCode(w, 400, createErrCode(err), err.Error())
 		return
 	}
 	writeJSON(w, 201, map[string]any{