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/ui_modal.rs

@@ -335,8 +335,7 @@ pub fn spawn_modal_button<M: Component>(
     variant: ButtonVariant,
     font_res: Option<&FontResource>,
 ) {
-    #[cfg(target_os = "android")]
-    let hotkey: Option<&'static str> = None;
+    let hotkey = if cfg!(target_os = "android") { None } else { hotkey };
     let font_handle = font_res.map(|f| f.0.clone()).unwrap_or_default();
     let font_label = TextFont {
         font: font_handle.clone(),