fix(plugin-api): position cursor after insertions and removals in prompt/attachments API

Ensure prompt.insertText, attachments.insertFileReference, and
attachments.removeFileReference move the cursor to the correct
position after modifying the prompt editor. Previously the cursor
stayed at the start of inserted text, which broke the natural
flow for plugin-driven file attachments like screenshot-paste.

src/client/src/components/PiWebApp.ts

@@ -1391,7 +1391,10 @@ export class PiWebApp extends LitElement {
         if (!editor) return;
         if (!editor.hasFocus) editor.focus();
         const sel = editor.state.selection.main;
-        editor.dispatch({ changes: { from: sel.from, to: sel.to, insert: text } });
+        editor.dispatch({
+          changes: { from: sel.from, to: sel.to, insert: text },
+          selection: { anchor: sel.from + text.length },
+        });
       },
       getText: () => {
         return this.promptEditor?.view?.state.doc.toString() ?? "";
@@ -1440,7 +1443,10 @@ export class PiWebApp extends LitElement {
         const editor = this.promptEditor?.view;
         if (editor) {
           const sel = editor.state.selection.main;
-          editor.dispatch({ changes: { from: sel.from, to: sel.to, insert: reference } });
+          editor.dispatch({
+            changes: { from: sel.from, to: sel.to, insert: reference },
+            selection: { anchor: sel.from + reference.length },
+          });
         }
         return reference;
       },
@@ -1463,6 +1469,7 @@ export class PiWebApp extends LitElement {
         if (index === -1) return;
         editor.dispatch({
           changes: { from: index, to: index + reference.length, insert: "" },
+          selection: { anchor: index },
         });
       },
     };