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.'
     }