[Feature] Add settings panel with volume control and display options

[alimaandev]

Problem\nPlayers have no way to adjust game settings. The sound toggle is in the HUD, but other settings (display mode, language, etc.) require page reload.\n\nSolution\nCreate a SettingsScreen component (src/components/SettingsScreen.tsx) with:\n- Volume slider (master gain)\n- Language selector (already exists in StartScreen — reuse)\n- Toggle for CRT/scanline overlay effect\n- Toggle for reduced motion\n- Close button → returns to previous screen\n\nMount it in Game.tsx when screen === 'settings'.\n\nDepends on: sound toggle (#8) already merged.