Replace the empty-prompt alert with an inline, context-aware hint

Clicking Write with AI with an empty scenario box used to throw a
blocking browser alert (especially jarring inside the HF Space iframe).
Now it focuses the prompt field and shows an inline hint — and when an
example script is already loaded, the hint points to Generate Audio
instead of implying something is wrong. Typing clears the hint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

static/app.js

@@ -629,10 +629,20 @@ el.loadScriptBtn.addEventListener("click", async () => {
 });
 
 /* ---------------- AI script generation ---------------- */
+const PROMPT_HINTS = [
+  "Describe a scenario above first.",
+  "Describe a new scenario above — or press Generate Audio to voice the script you already have.",
+];
+
+el.scriptPrompt.addEventListener("input", () => {
+  if (PROMPT_HINTS.includes(el.scriptGenStatus.textContent)) el.scriptGenStatus.textContent = "";
+});
+
 el.generateScriptBtn.addEventListener("click", async () => {
   const prompt = el.scriptPrompt.value.trim();
   if (!prompt) {
-    alert("Describe a scenario first.");
+    el.scriptPrompt.focus();
+    el.scriptGenStatus.textContent = state.turns.length ? PROMPT_HINTS[1] : PROMPT_HINTS[0];
     return;
   }