Editor opens in native windows (one per file)

Real BrowserWindow per edited file with its own RPC instance, deduped by
sessionId+path. The same Angular bundle boots an editor-only root via
preload globals. Native close isn't cancellable in electrobun, so the
editor mirrors its draft to bun; traffic-light closes get a native
Save/Discard prompt after the fact, and failed saves park the draft in
the edit-leftovers dir. In-window closes (Cmd+W, Close) use a styled
prompt; the quit guard now also counts unsaved editors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/file-actions.service.ts

@@ -105,22 +105,14 @@ export class FileActionsService {
       return;
     }
 
-    const res = await this.rpc.request.readFile({
-      sessionId: panel.sessionId(),
-      path: entry.path,
-    });
-    if (!res.ok) {
-      this.toast.error(res.error);
-      return;
-    }
-    this.dialogs.editor.set({
+    // built-in editor: its own native window (reads the file itself)
+    const res = await this.rpc.request.openEditor({
       sessionId: panel.sessionId(),
       path: entry.path,
       name: entry.name,
       sessionTitle: panel.title(),
-      content: res.content,
-      panel,
     });
+    if (!res.ok) this.toast.error(res.error);
   }
 
   rename(panel: PanelState): void {