feat(web): city presets for station settings

Add a City preset dropdown (Astana, Karaganda, Almaty, Bishkek,
Tashkent, Yekaterinburg, Omsk, Novosibirsk, Krasnoyarsk, Moscow,
Saint Petersburg) that fills altitude, coordinates and time zone from
average values. Manual entry and Custom remain; the matching preset is
auto-selected on load when coordinates match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/web_page.h

@@ -185,7 +185,11 @@ static const char INDEX_HTML[] PROGMEM = R"HTML(
     <summary>Station settings <span class="caret">Adjust</span></summary>
     <hr class="rule">
     <div class="form">
-      <div class="grid2" style="margin-top:18px">
+      <div style="margin-top:18px">
+        <label>City preset</label>
+        <select id="s-city"></select>
+      </div>
+      <div class="grid2" style="margin-top:14px">
         <div><label>Altitude &middot; metres</label><input id="s-alt" type="number" step="1"></div>
         <div><label>Time zone</label><select id="s-tz"></select></div>
         <div><label>Latitude</label><input id="s-lat" type="number" step="0.0001"></div>
@@ -382,15 +386,53 @@ function updateSetHand(){
 function tzLabel(min){var s=min<0?"-":"+";var a=Math.abs(min);var h=Math.floor(a/60);var m=a%60;return "UTC"+s+h+(m?(":"+(m<10?"0":"")+m):"");}
 function buildTz(){var sel=document.getElementById("s-tz");for(var m=-720;m<=840;m+=30){var o=document.createElement("option");o.value=m;o.textContent=tzLabel(m);sel.appendChild(o);}}
 
+// City presets: approximate average elevation (m), coordinates and UTC offset (min).
+var CITIES=[
+  {n:"Astana",          alt:347, lat:51.1694, lon:71.4491, tz:300},
+  {n:"Karaganda",       alt:553, lat:49.8047, lon:73.1094, tz:300},
+  {n:"Almaty",          alt:848, lat:43.2220, lon:76.8512, tz:300},
+  {n:"Bishkek",         alt:800, lat:42.8746, lon:74.5698, tz:360},
+  {n:"Tashkent",        alt:455, lat:41.2995, lon:69.2401, tz:300},
+  {n:"Yekaterinburg",   alt:237, lat:56.8389, lon:60.6057, tz:300},
+  {n:"Omsk",            alt:87,  lat:54.9885, lon:73.3242, tz:360},
+  {n:"Novosibirsk",     alt:150, lat:55.0084, lon:82.9357, tz:420},
+  {n:"Krasnoyarsk",     alt:137, lat:56.0153, lon:92.8932, tz:420},
+  {n:"Moscow",          alt:156, lat:55.7558, lon:37.6173, tz:180},
+  {n:"Saint Petersburg",alt:3,   lat:59.9311, lon:30.3609, tz:180}
+];
+function setTz(min){
+  var tz=document.getElementById("s-tz"); tz.value=String(min);
+  if(parseInt(tz.value,10)!==min){var o=document.createElement("option");o.value=min;o.textContent=tzLabel(min);tz.appendChild(o);tz.value=String(min);}
+}
+function buildCities(){
+  var sel=document.getElementById("s-city");
+  var o0=document.createElement("option"); o0.value="-1"; o0.textContent="Custom / manual entry"; sel.appendChild(o0);
+  CITIES.forEach(function(c,i){
+    var o=document.createElement("option"); o.value=i;
+    o.textContent=c.n+" · "+c.alt+" m · "+tzLabel(c.tz); sel.appendChild(o);
+  });
+  sel.addEventListener("change",function(){
+    var i=parseInt(sel.value,10); if(i<0) return;
+    var c=CITIES[i];
+    document.getElementById("s-alt").value=c.alt;
+    document.getElementById("s-lat").value=c.lat;
+    document.getElementById("s-lon").value=c.lon;
+    setTz(c.tz);
+  });
+}
+
 async function loadSettings(){
   try{
     var s=await (await fetch("/api/settings")).json();
     document.getElementById("s-alt").value=s.altitude;
-    var tz=document.getElementById("s-tz"); tz.value=String(s.tz);
+    setTz(s.tz);
-    if(parseInt(tz.value,10)!==s.tz){var o=document.createElement("option");o.value=s.tz;o.textContent=tzLabel(s.tz);tz.appendChild(o);tz.value=String(s.tz);}
     document.getElementById("s-lat").value=s.lat;
     document.getElementById("s-lon").value=s.lon;
     document.getElementById("coords").innerHTML=Number(s.lat).toFixed(2)+"&deg;N, "+Number(s.lon).toFixed(2)+"&deg;E";
+    // Auto-select a matching city preset, else "Custom".
+    var ci=-1;
+    for(var k=0;k<CITIES.length;k++){ if(Math.abs(CITIES[k].lat-s.lat)<0.02 && Math.abs(CITIES[k].lon-s.lon)<0.02){ ci=k; break; } }
+    document.getElementById("s-city").value=String(ci);
   }catch(e){}
 }
 
@@ -413,6 +455,7 @@ document.getElementById("save").addEventListener("click", async function(){
 
 buildGauge();
 buildTz();
+buildCities();
 loadSettings();
 refresh();
 setInterval(refresh,15000);