feat(gain): normalize gain controls across modes

- Pager and sensor gain inputs changed from unvalidated text fields to
  number inputs with min/max/step constraints
- ADS-B dashboard now exposes a gain input in the tracking strip;
  previously gain was hardcoded to 40 dB with no user control
- validate_gain() ceiling raised from 50 to 102 dB to support HackRF
  (LNA 40 + VGA 62 = 102 dB combined) and LimeSDR (73 dB)
- sdrCapabilities gain_max values corrected: HackRF 62→102, Airspy 21→45
- onSDRTypeChanged() now propagates gain_max to all mode gain inputs so
  HTML constraints match the selected SDR's actual range

Closes #162

templates/partials/modes/sensor.html

@@ -18,7 +18,7 @@
         <h3>Settings</h3>
         <div class="form-group">
             <label for="sensorGain">Gain (dB, 0 = auto)</label>
-            <input type="text" id="sensorGain" value="0" placeholder="0-49 or 0 for auto">
+            <input type="number" id="sensorGain" value="0" min="0" max="50" step="0.5" placeholder="0 = auto">
         </div>
         <div class="form-group">
             <label for="sensorPpm">PPM Correction</label>