fix(a11y): Toggle aria-invalid briefly when async onChange rejects

When an async onChange handler rejects, the toggle snapped back to its
previous value silently — screen reader users had no signal that the
action failed, only that the toggle suddenly returned to its original
state. Now aria-invalid="true" exposes the failure on the role=switch
button for 1.5s after rejection, so assistive tech announces the error.
Self-clears via setTimeout; no API change for callers.

src/lib/components/Toggle.svelte

@@ -17,6 +17,10 @@
   // Kept separate from the `loading` prop so callers can also force loading externally.
   let pending = $state(false);
 
+  // Briefly true after an async onChange rejects, so screen readers hear that
+  // the action failed rather than silently inferring it from the snap-back.
+  let snapBack = $state(false);
+
   // Effective busy state covers both the externally-controlled prop and our internal pending flag.
   let busy = $derived(loading || pending);
 
@@ -41,7 +45,11 @@
           await result;
           checked = next;
         } catch (_) {
-          // Swallow; the toggle simply stays at its previous value.
+          // Swallow; the toggle stays at its previous value. Flag the snap-back
+          // briefly so assistive tech announces aria-invalid="true" and screen
+          // reader users hear that the action failed.
+          snapBack = true;
+          setTimeout(() => { snapBack = false; }, 1500);
         } finally {
           pending = false;
         }
@@ -68,6 +76,7 @@
     aria-label={label}
     aria-disabled={disabled ? "true" : undefined}
     aria-busy={busy ? "true" : undefined}
+    aria-invalid={snapBack ? "true" : undefined}
   >
     <span
       class="absolute top-[3px] left-[3px] w-4 h-4 rounded-full shadow-sm flex items-center justify-center