[feat](trx-frontend-http): show location grid and city/country in header

Add Maidenhead locator and reverse-geocoded city/country to the header.
Uses Nominatim API to resolve nearest city asynchronously.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -27,6 +27,7 @@
             <div class="title"><span id="rig-title">trx-rs</span></div>
             <div class="subtitle" id="server-subtitle"></div>
             <div class="subtitle" id="rig-subtitle" style="font-weight: 700;">Rig: --</div>
+            <div class="subtitle" id="location-subtitle" style="display:none;"></div>
           </div>
         </div>
         <div class="tab-bar-nav">