Sound design upgrade

[benSlaughter]

Current sounds use Web Audio API synthesis — functional but robotic. Warm, natural sounds would elevate the whole experience.

Replace with short audio samples (.mp3, <50KB each):

• Success: gentle chime or xylophone note
• Click/tap: soft wooden tap
• Correct answer: ascending two-note melody
• Wrong answer: gentle low tone (not harsh buzzer)
• Achievement unlock: triumphant little fanfare
• Pop (bubble games): satisfying bubble pop
• Celebration: short confetti/sparkle sound

Source: freesound.org (CC0 licence) or generate with a tool like sfxr/jsfxr.

Storage: public/sounds/ directory, loaded via new Audio().

Keep: The speakWord() speech synthesis — that's great as-is.