Adjust the Electron sidebar header to properly center the T3 wordmark
with the native window controls. The logo was sitting ~2px too low and
~8px too close to the traffic lights.

- Reduce wordmark container top margin from mt-2 (8px) to mt-1.5 (6px)
- Increase Electron header left padding from 82px to 90px for proper
  traffic light clearance
- Match the desktop update button margin to stay aligned

Closes #564