feat(travelers): TCA9535 充电检测 + 键盘背光 + isVbusIn 修复

- 新增 P1.1 CHARGE_DET 充电检测（高电平=充电中），轮询间隔 2s
- Power.cpp isCharging()/isVbusIn() 均使用 TCA9535_CHARGE_DET_PIN 分支
- 新增 P1.0 键盘背光（高电平点亮），按键时亮，5s 无操作自动熄灭
- 修复开机供电维持：POWER_EN 在 Wire.begin() 后立即锁定
- 修复 P1 config 寄存器值 0x0A（之前 0x8D 导致 P1.2 高阻断电）
- ⚠️ 已知问题：TP4057 电压反串导致未充电时 P1.1 仍读高，需硬件修改

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