Web: rework MQTT IATA field

Replace the hardcoded 24-city <select id="mqttIata"> with an
<input list="mqttIataList"> + <datalist id="mqttIataList"> pair.
The datalist is populated at runtime from the same conf.json endpoint
used for radio presets, so the city list stays in sync with the API
without requiring firmware updates.

The <input> allows the user to type any IATA code directly, including
codes not present in the preset list. onfocus="this.select()" selects
all text on focus so the user can immediately type to replace the
current value without manually clearing it first.

Disable the Save button in HTML and enable it only when the field is
non-empty. The <input> event handler updates the button state as the
user types so Save is disabled immediately when the field is cleared.
refreshMqttIataWarning() also updates the button state after a Refresh
or Save, ensuring the button is re-enabled when the device value is
loaded.

The warning banner and inline warning are not updated while
the user is typing — the <input> event handler does not call
refreshMqttIataWarning(). The warning only reflects the device's
saved state, updated on Refresh and after a successful Save.

Switch the event listener from "change" to "input" so the Save
button responds to every keystroke rather than only on blur.

src/helpers/web/WebPanelServer.cpp

@@ -866,39 +866,10 @@ const char kWebPanelAppHtml[] PROGMEM = R"HTML(
         <div class="field-card">
           <label class="label" for="mqttIata">MQTT IATA</label>
           <div class="inline-actions">
-            <select id="mqttIata">
-              <optgroup label="Configuration">
-                <option value="UNSET">UNSET - To be configured</option>
-              </optgroup>
-              <optgroup label="Russia">
-                <option value="BSK">BSK - Biysk</option>
-                <option value="CEE">CEE - Cherepovets</option>
-                <option value="CEK">CEK - Chelyabinsk</option>
-                <option value="GOJ">GOJ - Nizhny Novgorod</option>
-                <option value="IKT">IKT - Irkutsk</option>
-                <option value="IWA">IWA - Ivanovo</option>
-                <option value="KHV">KHV - Khabarovsk</option>
-                <option value="KLD">KLD - Tver</option>
-                <option value="KLF">KLF - Kaluga</option>
-                <option value="KVX">KVX - Kirov</option>
-                <option value="KZN">KZN - Kazan</option>
-                <option value="LPK">LPK - Lipetsk</option>
-                <option value="MOW">MOW - Moscow</option>
-                <option value="OVB">OVB - Novosibirsk</option>
-                <option value="ROV">ROV - Rostov-on-Don</option>
-                <option value="RZN">RZN - Ryazan</option>
-                <option value="SVX">SVX - Yekaterinburg</option>
-                <option value="TBW">TBW - Tambov</option>
-                <option value="TYA">TYA - Tula</option>
-                <option value="UFA">UFA - Ufa</option>
-                <option value="VOG">VOG - Volgograd</option>
-                <option value="VOZ">VOZ - Voronezh</option>
-                <option value="VSG">VSG - Lugansk</option>
-                <option value="VVO">VVO - Vladivostok</option>
-              </optgroup>
-            </select>
+            <input id="mqttIata" list="mqttIataList" placeholder="Specify IATA code (e.g. MOW)" onfocus="this.select()">
+            <datalist id="mqttIataList"></datalist>
             <button class="iconbtn" data-load-cmd="get mqtt.iata" data-load-input="mqttIata" title="Refresh MQTT IATA">&#8635;</button>
-            <button class="savebtn" data-prefix="set mqtt.iata " data-input="mqttIata">Save</button>
+            <button class="savebtn" data-prefix="set mqtt.iata " data-input="mqttIata" disabled>Save</button>
           </div>
         </div>
         <div class="field-card">
@@ -2150,6 +2121,8 @@ const char kWebPanelAppHtml[] PROGMEM = R"HTML(
     }
     function refreshMqttIataWarning() {
       const input = document.getElementById("mqttIata");
+      const saveBtn = document.querySelector('[data-input="mqttIata"][data-prefix]');
+      if (saveBtn) saveBtn.disabled = !input || !input.value.trim();
       const banner = document.getElementById("mqttIataBanner");
       const inlineWarning = document.getElementById("mqttBrokerWarning");
       const showWarning = !!(input && isUnsetMqttIata(input.value));
@@ -2402,6 +2375,13 @@ const char kWebPanelAppHtml[] PROGMEM = R"HTML(
         }
         selectEl.innerHTML = options.join("");
         syncRadioPresetUi();
+        const datalist = document.getElementById("mqttIataList");
+        const iataOptions = ['<option value="UNSET">UNSET - To be configured</option>'];
+        for (const entry of entries) {
+          if (!entry.code || !entry.name) continue;
+          iataOptions.push(`<option value="${escapeHtml(entry.code)}">${escapeHtml(entry.name)}</option>`);
+        }
+        datalist.innerHTML = iataOptions.join("");
       } catch (error) {
         radioPresetEntries = [];
         selectEl.innerHTML = '<option value="">Community presets unavailable</option>';
@@ -2453,9 +2433,12 @@ const char kWebPanelAppHtml[] PROGMEM = R"HTML(
 	    document.querySelectorAll("[data-cmd]").forEach((btn) => btn.onclick = () => runCommand(btn.dataset.cmd));
 	    document.querySelectorAll("[data-prefix]").forEach((btn) => btn.onclick = () => runPrefixed(btn.dataset.prefix, btn.dataset.input));
 	    document.querySelectorAll("[data-load-cmd]").forEach((btn) => btn.onclick = () => loadField(btn.dataset.loadCmd, btn.dataset.loadInput, btn.dataset.loadFormat));
-	    const mqttIataSelect = document.getElementById("mqttIata");
-	    if (mqttIataSelect) {
-	      mqttIataSelect.addEventListener("change", refreshMqttIataWarning);
+	    const mqttIataInput = document.getElementById("mqttIata");
+	    if (mqttIataInput) {
+	      mqttIataInput.addEventListener("input", () => {
+	        const saveBtn = document.querySelector('[data-input="mqttIata"][data-prefix]');
+	        if (saveBtn) saveBtn.disabled = !mqttIataInput.value.trim();
+	      });
 	    }
 	    async function setMeshcoretelMode(enabled) {
 	      if (enabled && getLetsmeshMode() === "both") {