feat(decky): visible branded Steam shortcut, one-tap client updates, fullscreen-page polish

- The "Punktfunk" shortcut is no longer hidden: it now ships committed
  artwork (grid/wide/hero/logo/icon, generated by scripts/gen-steam-art.py
  — a pure-stdlib SDF renderer drawing the lens mark + a monoline
  "punktfunk" wordmark) applied via SetCustomArtworkForApp /
  SetShortcutIcon. Existing installs are unhidden and re-arted once per
  ART_VERSION; relaunching the library entry streams to the last host.
- Updates cover the flatpak CLIENT too: check_update compares the
  user-scope installed commit against its remote, applyUpdate runs
  `flatpak update --user` first (awaited) and the plugin reinstall —
  which reloads the panel — last; docs spell out the sudo-less --user
  update ("sudo flatpak update" silently skips per-user installs).
- Fullscreen page: DialogButton stretches to 100% width in the gamepad
  UI, so the Stream/Pair/Refresh/… actions filled whole rows — sized to
  content + right-aligned now; the header drops its Update button (About
  tab + QAM banner keep the flow) and the back button gets a real 40px
  hit target.
- Settings: the disable-Steam-Input note also shows for Automatic — on a
  Deck that now forwards the built-in controller as a Steam Deck pad
  (paddles/trackpads/gyro), which needs Steam Input off for the shortcut.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/decky/README.md

@@ -17,7 +17,7 @@ the panel looks and feels native to Gaming Mode.
    fingerprint to cross-check against the host's log).
 2. **Pair** — for a host that requires it, a gamepad-navigable PIN keypad runs the SPAKE2 pairing
    ceremony headlessly, then remembers the host so future streams connect silently.
-3. **Stream** — launches fullscreen via a hidden Steam shortcut so gamescope focuses it.
+3. **Stream** — launches fullscreen via a branded "Punktfunk" Steam shortcut so gamescope focuses it.
 4. **Settings** — resolution / refresh / bitrate / gamepad type / host compositor / mic, written
    to the client's config.
 5. **About** — plugin version, an explicit "Check for updates" button, the setup-guide link, and