fix: decode tab-wisdom PNG icons from base64 to binary

Both tab-wisdom.png and tab-wisdom-active.png were stored as
base64-encoded text instead of binary PNG data, causing the
wisdom (智) tab icon to not display in WeChat mini-program.

- tab-wisdom.png: 10989 bytes base64 text -> 8239 bytes binary PNG
- tab-wisdom-active.png: 8351 bytes base64 text -> 6261 bytes binary PNG

Note: active icon decoded from truncated base64 (missing ~0.75 bytes
of final scanline, imperceptible at 50x50)