fix(client-linux): Deck Gaming Mode — auto pad type, real chrome-less fullscreen, leave-to-Gaming-Mode, colour bisect

- "Automatic" gamepad type resolves to the virtual Steam Deck pad on Deck
  hardware (env SteamDeck / DMI Jupiter|Galileo): the built-in 28DE:1205
  identity is invisible at Hello time — the Valve HIDAPI drivers run
  in-session only and Steam Input shadows the pad with its virtual X360 —
  so auto always fell through to Xbox 360. "steamdeck" is now also
  selectable in Settings.
- Chrome-less launches flatten the window CSS (border-radius/box-shadow)
  and fullscreen at startup: gamescope never ACKs the xdg fullscreen
  state, so adwaita kept the floating-CSD rounded corners + shadow
  visible over the stream.
- Gaming-Mode --connect launches quit on session end, so Steam ends the
  "game" and the Deck returns to Gaming Mode — previously the app popped
  to its own hosts page, stranding the user fullscreen and making the
  escape chord read as broken.
- The capture hint is controller-aware; the chromeless hint teaches the
  hold-chord ("hold L1+R1+Start+Select to leave") and a quick chord press
  re-flashes it.
- Colour bisect for the reported off-colours on the VAAPI dmabuf path:
  graphics offload defaults OFF under gamescope (a subsurface hands the
  NV12 CSC to the compositor), PUNKTFUNK_OFFLOAD=1|0 overrides, and each
  colour-signaling change logs whether GDK accepted the BT.709-narrow
  color state (fallback = GDK's BT.601 dmabuf default).

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

clients/linux/src/ui_settings.rs

@@ -16,7 +16,14 @@ const RESOLUTIONS: &[(u32, u32)] = &[
 ];
 /// `0` = the monitor's native refresh, resolved at connect.
 const REFRESH: &[u32] = &[0, 30, 60, 90, 120, 144, 165, 240];
-const GAMEPADS: &[&str] = &["auto", "xbox360", "dualsense", "xboxone", "dualshock4"];
+const GAMEPADS: &[&str] = &[
+    "auto",
+    "xbox360",
+    "dualsense",
+    "xboxone",
+    "dualshock4",
+    "steamdeck",
+];
 const COMPOSITORS: &[&str] = &["auto", "kwin", "wlroots", "mutter", "gamescope"];
 /// Codec setting values (persisted) paired with their display labels below.
 const CODECS: &[&str] = &["auto", "hevc", "h264", "av1"];
@@ -403,6 +410,7 @@ pub fn show(
             "DualSense",
             "Xbox One",
             "DualShock 4",
+            "Steam Deck",
         ],
     );
     let inhibit_row = adw::SwitchRow::builder()