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/replay_overlay.rs

@@ -1165,6 +1165,7 @@ fn keybind_footer_mode_text() -> &'static str {
 /// pause/resume, the ESC accelerator for stop, and the ← / →
 /// accelerators for paused single-move stepping. The footer never
 /// lists unimplemented keybinds (would lie to users).
+#[cfg(not(target_os = "android"))]
 fn keybind_footer_hint_text() -> &'static str {
     "[SPACE] pause/resume \u{00B7} [ESC] stop \u{00B7} [\u{2190}\u{2192}] step" // · separator
 }