Deckback v0.0.2

Added

• Optional host-side idle-nudge helper (just idle-nudge): keeps the screen on and prevents
  auto-suspend while a video plays — SteamOS Game Mode otherwise blanks and suspends it, and nothing
  inside the Flatpak sandbox can stop that. Gated on playback (menus still sleep) and doesn't block a
  deliberate power-button sleep. Self-uninstalls when Deckback is removed.
• Self-hosted Flatpak repo published to GitHub Pages, with one-click install via a .flatpakref and
  automatic background updates (flatpak update).
• Flathub submission scaffold under flathub/ (extra-data engine manifest, apply_extra,
  flathub.json, SUBMISSION.md, and just flathub-prep).
• Steam library artwork (capsule / hero / logo / header / icon) and scripts/steam_shortcuts.py to
  install it onto the non-Steam shortcut.
• Neutral synthetic screenshots and Flathub-grade AppStream metadata (screenshots, branding,
  keywords) in the catalogue entry.

Fixed

• Swapped X/Y face buttons: physical X now play/pauses. The control table used the positional
  BTN_NORTH/BTN_WEST codes, which are aliased to the labelled BTN_X/BTN_Y the Deck's pad
  actually reports — so X and Y were reversed (pressing Y did play/pause).