Add working sound effects and background music via Web Audio API

Synthesized audio system with 9 distinct SFX (click, success, warning,
danger, purchase, achievement, era transition, info) mapped to all game
notifications, plus generative ambient background music with chord
progressions. Adds SFX volume slider to settings alongside existing
music volume control. No audio files or npm dependencies needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/shared/src/types/gameState.ts

@@ -45,11 +45,13 @@ export type GameSpeed = 1 | 2 | 5;
 export interface GameSettings {
   soundEnabled: boolean;
   musicVolume: number;
+  sfxVolume: number;
 }
 
 export const INITIAL_SETTINGS: GameSettings = {
   soundEnabled: true,
   musicVolume: 0.5,
+  sfxVolume: 0.5,
 };
 
 export const SAVE_VERSION = 10;