New layout:
- Mode box (left): RX/TX indicator with vertical separator
- Title box (right): 'DongLoRa v{ver}' + radio settings (freq/bw/sf/cr)
- Centered info rows: packet counters + RSSI/SNR with labels
- Sparkline bar graph fills remaining height
- Idle state returns to splash screen (active screen only for RX/TX)
- Layout positions derived from display dimensions (h/w agnostic)
- RSSI history buffer doubled to 128 slots (~2 min at 1s/slot)
- Smart compact_count() formatting for large RX/TX counters (k/M)
