fix: CR encoding bug, display overflow, auto-reconnect script

- Fix CodingRate encoding: postcard uses 0-based variant index, not
  repr(u8) value. CR 4/5 is variant 0, not 1.
- Shorten RSSI/SNR display line to fit 21-char width
- Python script auto-detects DongLoRa by USB VID:PID (303a:1001),
  reconnects on device reset/disconnect, no more crash on RST

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/display/render.rs

@@ -120,14 +120,14 @@ pub fn dashboard(
                 .draw(target)
                 .ok();
 
-            // Row 3: RSSI + SNR
+            // Row 3: RSSI + SNR (compact to fit 21 chars)
             buf.clear();
             match (status.last_rssi, status.last_snr) {
                 (Some(rssi), Some(snr)) => {
-                    let _ = write!(buf, "RSSI:{} dBm  SNR:{} dB", rssi, snr);
+                    let _ = write!(buf, "{}dBm  SNR:{}dB", rssi, snr);
                 }
                 (Some(rssi), None) => {
-                    let _ = write!(buf, "RSSI:{} dBm", rssi);
+                    let _ = write!(buf, "{}dBm", rssi);
                 }
                 _ => {
                     let _ = write!(buf, "No signal");