Deckback v0.0.4

Added

• Self-update, notify-first (self_update_mode: notify | auto | off, default
  notify). Deckback now tells you when a new version is out instead of updating behind your back.
  A small "Update available" pill sits in the top-right corner with a ☰ hint — open it from
  the Menu button any time — and once per version an Update available card shows what's new, with
  colour-coded buttons: A Update now (green), B Not now (red), Y Ignore this version
  (yellow). Nothing installs until you press A; the update then applies the next time you open
  Deckback. The pill stays out of the way during video playback. Prefer silent updates? Set
  self_update_mode: "auto"; "off" disables it and you update from Desktop Mode. Updates touch
  only Deckback, from its own repo — no root, no password; keeping the runtime and other apps
  current is still a separate flatpak update. See .internal/findings/durable/self-update.md.