refactor(engine): audit and rationalize platform cfg gates (closes #49)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/home_plugin.rs

@@ -174,17 +174,17 @@ impl HomeMode {
     }
 
     /// The keyboard accelerator that dispatches the same launch event,
-    /// shown in a small chip on the card.
-    #[cfg(not(target_os = "android"))]
-    fn hotkey(self) -> &'static str {
-        match self {
+    /// shown in a small chip on desktop cards.
+    fn hotkey(self) -> Option<&'static str> {
+        let key = match self {
             HomeMode::Classic => "N",
             HomeMode::Daily => "C",
             HomeMode::Zen => "Z",
             HomeMode::Challenge => "X",
             HomeMode::TimeAttack => "T",
             HomeMode::PlayBySeed => "6",
-        }
+        };
+        crate::platform::SHOW_KEYBOARD_ACCELERATORS.then_some(key)
     }
 
     /// `true` when the mode is gated behind `CHALLENGE_UNLOCK_LEVEL`.
@@ -1392,27 +1392,28 @@ fn spawn_mode_card(
                 ));
 
                 if unlocked {
-                    // Hotkey chip — suppressed on Android (touch builds have no keyboard).
-                    #[cfg(not(target_os = "android"))]
-                    row.spawn((
-                        Node {
-                            padding: UiRect::axes(VAL_SPACE_2, VAL_SPACE_1),
-                            min_width: Val::Px(32.0),
-                            justify_content: JustifyContent::Center,
-                            border: UiRect::all(Val::Px(1.0)),
-                            border_radius: BorderRadius::all(Val::Px(RADIUS_MD)),
-                            ..default()
-                        },
-                        BorderColor::all(BORDER_SUBTLE),
-                        HighContrastBorder::with_default(BORDER_SUBTLE),
-                    ))
-                    .with_children(|chip| {
-                        chip.spawn((
-                            Text::new(mode.hotkey().to_string()),
-                            font_chip.clone(),
-                            TextColor(TEXT_SECONDARY),
-                        ));
-                    });
+                    // Hotkey chip — suppressed on touch-first Android builds.
+                    if let Some(hotkey) = mode.hotkey() {
+                        row.spawn((
+                            Node {
+                                padding: UiRect::axes(VAL_SPACE_2, VAL_SPACE_1),
+                                min_width: Val::Px(32.0),
+                                justify_content: JustifyContent::Center,
+                                border: UiRect::all(Val::Px(1.0)),
+                                border_radius: BorderRadius::all(Val::Px(RADIUS_MD)),
+                                ..default()
+                            },
+                            BorderColor::all(BORDER_SUBTLE),
+                            HighContrastBorder::with_default(BORDER_SUBTLE),
+                        ))
+                        .with_children(|chip| {
+                            chip.spawn((
+                                Text::new(hotkey),
+                                font_chip.clone(),
+                                TextColor(TEXT_SECONDARY),
+                            ));
+                        });
+                    }
                 } else {
                     // Lock icon stand-in — text glyph keeps the layout
                     // dependency-free (no asset loader required) and