fix(engine): silence Android-target dead-code and unused-import warnings

All 10 warnings were caused by hotkey/keyboard UI code behind
#[cfg(not(target_os = "android"))] call sites whose definitions lacked
the matching gate. Fixes:
- help_plugin: gate keyboard-chip imports and font_kbd; #[allow(dead_code)]
  on ControlRow (keys field is data, not dead)
- hud_plugin/ui_modal: replace cfg shadow pattern with cfg!() expression
  so the hotkey parameter is read on every platform
- home_plugin: gate fn hotkey behind not(android)
- onboarding_plugin: gate HotkeyRow, HOTKEYS, spawn_slide_hotkeys and
  their exclusive imports behind not(android)
- replay_overlay: gate keybind_footer_hint_text behind not(android)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/hud_plugin.rs

@@ -799,8 +799,7 @@ fn spawn_action_button<M: Component>(
     // visibly clutter the narrow-viewport action row. Force the hint
     // off on Android; the chevrons on Menu/Modes remain because they
     // indicate dropdown behaviour and still apply on touch.
-    #[cfg(target_os = "android")]
-    let hotkey: Option<&'static str> = None;
+    let hotkey = if cfg!(target_os = "android") { None } else { hotkey };
 
     let hotkey_font = TextFont {
         font: font.font.clone(),