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_test.go

@@ -0,0 +1,24 @@
+package logic
+
+import "testing"
+
+func TestHasRequiredPromptTextAllowsInlineReferenceContext(t *testing.T) {
+	prompt := "参考图 1（0.jpg）：http://localhost:19000/canvas-assets/a.webp 为服装品牌 stillday 创建情绪板 logo 设计 参考图 2（image.png）：http://localhost:19000/canvas-assets/b.png ，包含材质、色彩、摄影姿态。"
+	if !hasRequiredPromptText(prompt) {
+		t.Fatalf("expected inline text around reference directives to count as prompt text")
+	}
+}
+
+func TestHasRequiredPromptTextRejectsReferenceOnlyPrompt(t *testing.T) {
+	prompt := "参考图 1（0.jpg）：http://localhost:19000/canvas-assets/a.webp\nReference image 2 (image.png): https://example.com/b.png"
+	if hasRequiredPromptText(prompt) {
+		t.Fatalf("expected reference-only prompt to be treated as empty")
+	}
+}
+
+func TestHasRequiredPromptTextHandlesChinesePunctuationAfterReference(t *testing.T) {
+	prompt := "参考图 1（image.png）：https://example.com/reference.png，参考它的字体做 logo。"
+	if !hasRequiredPromptText(prompt) {
+		t.Fatalf("expected text after Chinese punctuation to count as prompt text")
+	}
+}