feat(audio): add the Audio source dropdown to the welcome screen

The opening screen only had Volume + Video speed — no way to pick the sound mode
before launching. Add a welcome Audio dropdown (None/Soundtrack/Music) mirroring
the panel; applyWelcomeToPanel carries the choice into the session on launch, None
greys the welcome Volume, and ?audio= presets it on the welcome screen too. +4 e2e
(welcome-audio.spec.ts).

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

simulator/static/app.js

@@ -1420,6 +1420,7 @@ function applyUrlAudioParams() {
   const src = (p.get("audio") || "").toLowerCase();
   if (src === "none" || src === "soundtrack" || src === "music") {
     const el = $("audio-source"); if (el) el.value = src;
+    const w = $("welcome-audio-source"); if (w) w.value = src;   // reflect on the opening screen too
   }
   const vol = p.get("vol");
   if (vol !== null && vol !== "" && Number.isFinite(+vol)) {
@@ -1681,11 +1682,18 @@ function applyWelcomeToPanel() {
   $("visual").checked = true;   // the welcome screen assumes Video on (no toggle)
   const wa = $("welcome-audio");
   if (wa) { $("audio").value = wa.value; updateAudioLevelLabel(); }
+  const ws = $("welcome-audio-source");   // carry the chosen source into the panel dropdown
+  if (ws) { $("audio-source").value = ws.value; syncVolumeDisabled(); }
 }
 function updateWelcomeAudioLabel() {
   const el = $("welcome-audio-val");
   if (el) el.textContent = String(+($("welcome-audio") || {}).value || 0);
 }
+// Volume is meaningless on None — grey the welcome slider too, mirroring the panel.
+function syncWelcomeVolumeDisabled() {
+  const src = $("welcome-audio-source"), slider = $("welcome-audio");
+  if (src && slider) slider.disabled = src.value === "none";
+}
 
 // Remove the welcome overlay (fade out) — the panel reveals via `body:has(#welcome)`.
 function enterSimulation() {
@@ -1718,6 +1726,9 @@ function initWelcome() {
   updateWelcomeAudioLabel();
   const wa = $("welcome-audio");
   if (wa) wa.addEventListener("input", updateWelcomeAudioLabel);
+  const ws = $("welcome-audio-source");
+  if (ws) ws.addEventListener("change", syncWelcomeVolumeDisabled);
+  syncWelcomeVolumeDisabled();
   const launch = $("welcome-launch");
   if (launch) launch.addEventListener("click", onLaunch);
 }
@@ -1748,6 +1759,7 @@ async function main() {
   $("audio-source").addEventListener("change", () => { syncVolumeDisabled(); applyAudio(); debounced(); });
   applyUrlAudioParams();        // shareable ?audio=/?vol= deep-link presets
   syncVolumeDisabled();
+  syncWelcomeVolumeDisabled();  // reflect a URL-set source on the welcome slider too
   updateWelcomeAudioLabel();
   // Video toggle (panel): the FIRST time video turns on without having entered via
   // Launch, begin the experience too — couple audio in. (Normally Launch entered

simulator/static/index.html

@@ -26,6 +26,13 @@
                  list="play-speed-ticks" aria-describedby="welcome-play-speed-val" />
           <span id="welcome-play-speed-val">1.00×</span>
         </label>
+        <label class="audio-source"><span data-i18n="output.audio">Audio</span>
+          <select id="welcome-audio-source" aria-label="Audio source">
+            <option value="none" data-i18n="output.audio.none">None</option>
+            <option value="soundtrack" data-i18n="output.audio.soundtrack" selected>Soundtrack</option>
+            <option value="music" data-i18n="output.audio.music">Music</option>
+          </select>
+        </label>
         <label class="audio-level"><span data-i18n="output.volume">Volume</span>
           <input type="range" id="welcome-audio" min="0" max="10" value="2" step="1" />
           <span id="welcome-audio-val">2</span>/10