feat: redesign tab icons to match 五行沙盘 emoji themes

Redrew all 5 tab icons to visually match the sandbox emojis:
- 身(土): 🌏 globe with meridian and continent arcs
- 智(金): ⚡ lightning bolt shape
- 富(水): 💧 water drop with pointed top
- 行(木): 🌿 leaf with stem and veins
- 心(火): 🔥 flame with inner/outer layers

All 81x81px, gray (#999) normal + element color active.