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/launch.rs

@@ -266,6 +266,9 @@ impl SessionUi {
             inhibit_shortcuts: self.inhibit,
             show_stats: self.show_stats,
             chromeless: self.app.fullscreen,
+            // The attach just went out, so a Deck's built-in pad may not have enumerated
+            // yet — chromeless (controller-first) shows the chord hint regardless.
+            pad_connected: self.app.gamepad.active().is_some(),
             title,
         });
         self.app.nav.push(&p.page);
@@ -311,6 +314,16 @@ impl SessionUi {
     fn on_ended(&mut self, err: Option<String>) {
         self.close_waiting();
         self.app.gamepad.detach();
+        // Gaming-Mode `--connect` launch: the app IS the stream. Quit so Steam ends the
+        // "game" and the Deck returns to Gaming Mode — popping to our own hosts page would
+        // strand the user in a fullscreen shell with no way back.
+        if self.app.quit_on_session_end {
+            if let Some(e) = err {
+                tracing::warn!(error = %e, "session ended");
+            }
+            self.app.window.close();
+            return;
+        }
         self.app.nav.pop_to_tag("hosts");
         if let Some(h) = self.app.hosts_ui() {
             h.set_connecting(None);