Fix AI edit mode: strip code fences, stronger prompt

internal/api/ai.go

@@ -157,10 +157,11 @@ func (s *Server) handleAIChat(w http.ResponseWriter, r *http.Request) {
 	var userMsg string
 
 	if req.Mode == "edit" {
-		systemPrompt = `You are a document editor. The user will give you a markdown document and an instruction.
-Apply the instruction to the document and return ONLY the complete updated document.
-Do not add explanations, comments, or markdown code fences around the output.
-Preserve the document's existing style and formatting.`
+		systemPrompt = "You are a document editor. The user will give you a markdown document and an instruction. " +
+			"Apply the instruction to the document and return ONLY the complete updated document. " +
+			"Do not add explanations, comments, or wrap the output in code fences. " +
+			"Do not prefix with any markers. Preserve the document's existing style and formatting. " +
+			"Return the raw markdown content only."
 		userMsg = "Document:\n\n" + req.Content + "\n\nInstruction: " + req.Message
 	} else {
 		systemPrompt = `You are a helpful writing assistant. The user has a markdown document open and is asking a question about it.
@@ -175,6 +176,18 @@ Answer concisely in markdown. Reference the document content when relevant.`
 	}
 
 	if req.Mode == "edit" {
+		// Strip markdown code fences if AI wrapped the output
+		response = strings.TrimSpace(response)
+		if strings.HasPrefix(response, "```") {
+			lines := strings.Split(response, "\n")
+			if len(lines) > 2 {
+				lines = lines[1:] // remove opening fence
+				if strings.TrimSpace(lines[len(lines)-1]) == "```" {
+					lines = lines[:len(lines)-1] // remove closing fence
+				}
+				response = strings.Join(lines, "\n")
+			}
+		}
 		writeJSON(w, 200, map[string]string{"content": response})
 	} else {
 		writeJSON(w, 200, map[string]string{"result": response})