feat(desktop): build a Linux .deb alongside the AppImage (#5078)

Adds @electron-forge/maker-deb so `electron-forge make` on Linux emits a
.deb next to the existing AppImage.

- forge.config: MakerDeb scoped to linux; `bin` pinned to packagerConfig.name
  so the staging build ("Folo Staging") also resolves; folo:// and follow://
  registered as x-scheme-handler MIME types; Network;News; categories.
- postMake: .deb is renamed to the Folo-<version>-linux-<arch>.deb convention
  but kept out of latest-linux.yml (the electron-updater feed is AppImage-only).
- CI: install fakeroot/dpkg on the Linux leg; upload the .deb as its own
  `linux-x64-deb` artifact; add *.deb to the attestation and release globs.
- pnpm-lock.yaml regenerated with the pinned pnpm@10.17.0. The large churn is
  peer-key reserialization from a newer pnpm than last wrote the lockfile;
  the only functional additions are maker-deb and its electron-installer-debian
  subtree.


Claude-Session: https://claude.ai/code/session_01KeQJNfE1FfJnLMH9psG1jS

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>