feat: circuit board + blast door posts, signal packet bsky, fix fonts

- Posts alternate between circuit board (PCB grid, trace lines, chip
  frame, pin notches) and blast door (hex bolt rail, bevel, warning
  stripe) with normal-sized readable titles
- Bsky tiles switch from comm ping to signal packet (stepped corners,
  dashed borders, structured header with field dividers, hex checksum)
- Mono font stack starts from ui-monospace (removed Nimbus Mono PS)
- Body font stays system-ui