fix(desktop): let Files panel download remote backend files

[xxxigm]

Summary

• On a remote gateway, the Desktop Files panel only offered Copy Path / Copy Relative Path. Those are server paths, so they do not put the file on the local machine.
• Add a Download action for files (not folders) in remote mode, using the existing authenticated gateway save bridge (saveGatewayFile / /api/fs/download).
• Local Reveal / Rename / Delete stay hidden in remote mode; copy-path still works.

Test plan

• npm run test --workspace apps/desktop -- src/store/file-actions.test.ts src/app/right-sidebar/review/file-tree.test.tsx
• Connect Desktop (Windows) to a Linux remote gateway, right-click a file in the Files panel, choose Download, and confirm the save dialog writes it locally
• Cancel the save dialog — no toast, no local file
• Right-click a folder in remote mode — Download is absent
• Local (non-remote) Files panel still shows Reveal / Rename / Delete and no Download

[OutThisLife]

LGTM — Download for remote Files panel files is the right fix, and it reuses the existing gateway save bridge. Thanks @xxxigm.