AI edit: return only changes, not full document

- Edit prompt asks for APPEND/PREPEND/REPLACE + only new content
- Frontend applies the edit surgically (append/prepend/replace)
- Much faster: AI only generates the new text, not the whole doc
- Chat mode still streams response progressively

frontend/src/App.vue

@@ -1038,9 +1038,7 @@ async function sendAiChat() {
         try {
           const token_text = JSON.parse(data)
           fullText += token_text
-          if (action === 'edit') {
-            content.value = fullText
-          } else {
+          if (action !== 'edit') {
             aiChatResponse.value = fullText
           }
         } catch {}
@@ -1048,6 +1046,19 @@ async function sendAiChat() {
     }
 
     if (action === 'edit') {
+      // Parse location hint from AI response
+      const firstNewline = fullText.indexOf('\n')
+      const hint = firstNewline > -1 ? fullText.substring(0, firstNewline).trim().toUpperCase() : ''
+      const editContent = firstNewline > -1 ? fullText.substring(firstNewline + 1) : fullText
+
+      if (hint === 'APPEND') {
+        content.value = content.value.trimEnd() + '\n\n' + editContent
+      } else if (hint === 'PREPEND') {
+        content.value = editContent + '\n\n' + content.value
+      } else {
+        // REPLACE or unknown — replace entire document
+        content.value = hint === 'REPLACE' ? editContent : fullText
+      }
       isDirty.value = true
       aiChatResponse.value = 'Document updated.'
     }

internal/api/ai.go

@@ -158,8 +158,11 @@ func (s *Server) handleAIChat(w http.ResponseWriter, r *http.Request) {
 	var systemPrompt, 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 and return the COMPLETE updated document. " +
-			"Do not add explanations or wrap in code fences. Return raw markdown only."
+			"Return ONLY the new or changed text that should be inserted/replaced. " +
+			"Do not return the entire document. Do not add explanations or wrap in code fences. " +
+			"If adding content, return only the addition. If modifying, return only the modified section. " +
+			"Start your response with a location hint on the first line: APPEND (add to end), PREPEND (add to start), or REPLACE (replace entire document). " +
+			"Then the content on the next line."
 		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. Answer concisely in markdown."