fix(logic): ignore reference directives when checking for prompt text

Strip inline image tokens and reference directive URLs before deciding
whether a prompt carries real user text, so a reference-only prompt is
still treated as empty while text around references counts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/prompt_validation.go

@@ -2,11 +2,17 @@ package logic
 
 import (
 	"fmt"
+	"regexp"
 	"strings"
 
 	"img_infinite_canvas/internal/domain/design"
 )
 
+var (
+	promptReferenceDirectivePattern = regexp.MustCompile(`(?i)(?:参考图|reference image)\s*\d*\s*(?:[（(][^）)\n]+[）)])?\s*[:：]\s*(?:https?://[^\s\]，。]+|data:image/[^\s\]，。]+|/[^\s\]，。]+)`)
+	promptImageTokenPattern         = regexp.MustCompile(`\[@image:#\d+:[^:\]]+:(?:\[[^\]]+\]\((?:[^)]+)\)|[^\]]+)\]`)
+)
+
 func validateRequiredPrompt(prompt string, allowEmpty bool) error {
 	if allowEmpty {
 		return nil
@@ -18,6 +24,8 @@ func validateRequiredPrompt(prompt string, allowEmpty bool) error {
 }
 
 func hasRequiredPromptText(prompt string) bool {
+	prompt = promptImageTokenPattern.ReplaceAllString(prompt, " ")
+	prompt = promptReferenceDirectivePattern.ReplaceAllString(prompt, " ")
 	for _, line := range strings.Split(prompt, "\n") {
 		line = strings.TrimSpace(strings.ReplaceAll(line, "\u00a0", " "))
 		if line == "" || isPromptReferenceDirective(line) {