[feat](trx-frontend-http): add clickable Sig Strength field between Wavelength and Frequency

Add a new "Sig Strength" display field in the freq row that shows
the measured signal strength. Clicking the field cycles through
three units: dBFS (default), dBf, and dBm. The selected unit is
persisted in localStorage.

https://claude.ai/code/session_01EvRV8UgsVtbrcH4t2hmFBF
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -176,6 +176,10 @@
             <div class="wavelength-display" id="wavelength" title="Wavelength">--</div>
             <div class="label"><span>Wavelength</span></div>
           </div>
+          <div class="freq-field sig-strength-col">
+            <div class="sig-strength-display" id="sig-strength" title="Click to change unit">--</div>
+            <div class="label"><span>Sig Strength</span></div>
+          </div>
           <div class="freq-field frequency-col">
             <input class="status-input" id="freq" type="text" value="--" />
             <div class="label"><span>Frequency</span></div>