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/ratelimit.go

@@ -113,7 +113,7 @@ func rateLimitAdmin(r *http.Request) bool {
 // writeRateLimited responds 429 with Retry-After based on refill rate.
 func writeRateLimited(w http.ResponseWriter, retryAfterSecs int) {
 	w.Header().Set("Retry-After", strconv.Itoa(retryAfterSecs))
-	writeErr(w, 429, "rate limit exceeded")
+	writeErrCode(w, 429, "rate_limited", "rate limit exceeded")
 }
 
 // setRateLimitHeaders sets informational X-RateLimit headers for create/guess.