Web: reorder Radio HUD air-time metrics (RX first, TX Share last)

Reorder the four air-time metrics in the Radio card:

  Before: TX Air  | RX Air
          TX Share | Total Air

  After:  RX Air    | TX Air
          Total Air | TX Share

Move RX Air to the primary (top-left) position — a repeater
receives before it retransmits, so RX is the primary signal.
Move TX Share after Total Air so the derived percentage follows
the raw counters it summarises.

src/helpers/web/WebPanelServer.cpp

@@ -1540,10 +1540,10 @@ const char kWebPanelAppHtml[] PROGMEM = R"HTML(
         ${renderMeter("SNR", Number.isFinite(radio.last_snr) ? radio.last_snr.toFixed(1) + " dB" : "--", snrPct, "link quality", snrTone)}
         ${renderMeter("Noise Floor", (radio.noise_floor ?? "--") + " dBm", noisePct, "ambient RF", noiseTone)}
         <div class="metric-grid">
-          ${renderMetric("TX Air", String(radio.tx_air_secs ?? 0) + " s")}
           ${renderMetric("RX Air", String(radio.rx_air_secs ?? 0) + " s")}
-          ${renderMetric("TX Share", Math.round(txShare) + "%")}
+          ${renderMetric("TX Air", String(radio.tx_air_secs ?? 0) + " s")}
           ${renderMetric("Total Air", String(totalAir) + " s")}
+          ${renderMetric("TX Share", Math.round(txShare) + "%")}
         </div>
       </section>`;
     }