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/help_plugin.rs

@@ -13,10 +13,9 @@ use crate::ui_modal::{
     spawn_modal, spawn_modal_actions, spawn_modal_button, spawn_modal_header, ButtonVariant,
     ScrimDismissible,
 };
-use crate::ui_theme::{
-    BORDER_SUBTLE, HighContrastBorder, RADIUS_SM, SPACE_2, TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY,
-    TYPE_CAPTION, VAL_SPACE_1, VAL_SPACE_2, VAL_SPACE_3, Z_MODAL_PANEL,
-};
+use crate::ui_theme::{SPACE_2, TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY, VAL_SPACE_2, VAL_SPACE_3, Z_MODAL_PANEL};
+#[cfg(not(target_os = "android"))]
+use crate::ui_theme::{BORDER_SUBTLE, HighContrastBorder, RADIUS_SM, TYPE_CAPTION, VAL_SPACE_1};
 
 /// Marker on the help overlay root node.
 #[derive(Component, Debug)]
@@ -123,6 +122,7 @@ fn scroll_help_panel(
 }
 
 /// Each entry in the controls reference table.
+#[allow(dead_code)]
 struct ControlRow {
     keys: &'static str,
     description: &'static str,
@@ -243,6 +243,7 @@ fn spawn_help_screen(commands: &mut Commands, font_res: Option<&FontResource>) {
         ..default()
     };
     let font_row = font_section.clone();
+    #[cfg(not(target_os = "android"))]
     let font_kbd = TextFont {
         font: font_handle,
         font_size: TYPE_CAPTION,