feat: add ujust get-couchside (phone remote & dashboard for the couch)

[emerytech]

Adds a get-couchside recipe to 82-bazzite-apps.just, following the
get-emudeck shape exactly: status | install | uninstall plus the
interactive ugum menu when run bare. No yafti/portal change in this PR —
recipe only.

What Couchside is

Couchside turns a phone (iOS/Android) into the
dashboard, remote console, and game controller for a living-room Bazzite/
SteamOS machine: launch games, switch Game Mode ↔ Desktop, trackpad +
keyboard input, volume/TV control, power. LAN-only, bearer-token
authenticated, no cloud, no accounts, no analytics. The box-side agent is
MIT licensed, pure Python 3 stdlib, and runs as the desktop user (not root).

Why this shape

• Same install mechanism as Decky Loader's recipe: curl | bash of an
  installer that sets up a systemd service. The installer verifies an
  Ed25519 signature over every fetched file (SHA256SUMS + SHA256SUMS.sig
  against a pinned public key) before installing anything — a corrupted or
  tampered fetch refuses to install. Verification code is in the installer
  itself: https://github.com/emerytech/couchside/blob/main/install.sh
• status checks the systemd unit + installed daemon — what the installer
  actually creates, not a filename heuristic.
• uninstall is complete and testable (ujust get-couchside uninstall):
  service, privileged helper + its units, udev rules, sudoers (prompted),
  Steam tile, Wake-on-LAN link.

Anticipating the review questions

• Privileged surface: the agent executes only a frozen allowlist — client
  input selects entries, never becomes a command/path/shell string. Since
  2.9.69 the root-side work goes through a small socket-gated helper (eight
  fixed verbs, SO_PEERCRED-checked) rather than broad sudo rules.
• Blast radius: it can reboot/suspend the machine and switch sessions —
  that is the product. It is reachable only on the LAN and only with the
  pairing token the box displays.
• Maintenance: I maintain the recipe; the project ships weekly and the
  installer URL is stable (it is a signed release asset mirrored at
  couchside.tv).

Happy to adjust naming, grouping, or scope to fit — and no hard feelings if
the portal bar is "come back with more users"; the recipe alone already
gives Bazzite users a one-liner.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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