feat(engine): group HUD menu popover into Play/You/Community/System sections

Phase C of docs/ui-redesign-2026-07.md. The Modes row is gone — Home
owns mode selection, so the popover's Play section carries a Home row
firing the new ToggleHomeRequestEvent (read by toggle_home_screen
alongside the existing M accelerator). Section headers are quiet
caption-size labels inside the existing panel widget, not a new
widget. The action-bar Modes button and its popover are untouched
(their removal is Phase B territory when Home gains hierarchy).

- MenuOption: Modes variant replaced by Home; rows grouped Play (Home)
  · You (Profile, Stats, Achievements) · Community (Leaderboard) ·
  System (Settings, Help)
- handle_menu_option_click no longer chains into spawn_modes_popover
- Tests: tooltip sweep updated (7 rows still), new
  toggle_home_event_opens_home_screen covers the popover's open path

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/hud_plugin/tests.rs

@@ -786,8 +786,8 @@ fn popover_rows_carry_tooltip_strings() {
         menu_tooltips.len()
     );
     for expected in [
+        "Pick a mode, continue, or start a new game.",
         "Show controls, rules, and keyboard shortcuts.",
-        "Switch modes: Classic, Daily, Zen, Challenge, Time Attack.",
         "Lifetime totals: wins, streaks, fastest time, best score.",
         "Browse unlocked achievements and the rewards still ahead.",
         "Your level, XP progress, and sync status.",