The pause indicator was showing 'paused' while the game was still running
because isManualPaused was being modified by both user actions (clicking
pause button) and system actions (opening menu).
Changes:
- systemStore: pause()/resume() no longer modify isManualPaused, only
togglePause() does (with optimistic update + rollback on failure)
- useGameControl: consolidate 3 overlapping watches into 1 clean watch
that only handles menu open/close without polluting manual pause state
- App.vue: explicitly call resumeGame() API when game initializes
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