Fix: A/B slot toggle had no visible active state

The button rendered class.active but not the data-slot attribute the CSS
selector keyed off (.slot-btn[data-slot="A"].active), so the colored
background never applied and both buttons looked identical. Add data-slot
+ aria-pressed for assistive tech.

Author: teolbb

src/ui/App.ts

@@ -53,7 +53,8 @@ const shortLabel = (n: string): string => CURVE_SHORT[n] ?? n;
 const SlotToggle = (): Renderable => html`
   <div class="slot-toggle" role="tablist">
     ${(["A", "B"] as const).map((slot) => html`
-      <button type="button"
+      <button type="button" data-slot=${slot}
+              aria-pressed=${activeSlot.value === slot ? "true" : "false"}
               class=${`slot-btn ${activeSlot.value === slot ? "active" : ""}`}
               onClick=${() => (activeSlot.value = slot)}>${slot}</button>`)}
   </div>`;