feat: add file download for remote server connections #63485

Feature request

When connected to a remote OpenCode server, right-clicking a file in the file tree shows "Open in app" options that don't work (there's no local app to open remote files). Instead, show a Download option so users can save remote files locally.

Proposed approach

Add a downloadFile helper (packages/gui-plugins/src/file/download.ts) that loads file content through the session screen's Files API and saves it locally — text files via ctx.system.save, binary files via a Blob download.

Add a DownloadContextMenu component in open-in-app.tsx that replaces the "open in app" menu when no local app can open files (e.g. on a remote server). Wire it through list.tsx and pierre-tree.tsx by passing the screen prop to OpenInAppContextMenuV2.