feat(toggle): add disabled, loading, and async onChange props

Extends the shared Toggle with three optional props while keeping the
existing bind:checked path untouched for current call sites.

- disabled: dims the control, sets aria-disabled, and early-returns on click.
- loading: forces a Loader2 spinner inside the thumb, applies cursor-wait
  and aria-busy, and blocks interaction. Cursor-wait wins over disabled's
  not-allowed when both are set, matching loading-precedence guidance.
- onChange(next): when supplied, replaces the immediate flip. Promise
  returns drive an internal pending flag so the toggle renders loading
  while the handler resolves; on resolve checked flips, on reject the
  toggle snaps back. Synchronous handlers flip immediately after the call.

Brand motion preserved: 150ms duration, cubic-bezier(0.2, 0.8, 0.2, 1),
no transition-all.

src/lib/components/Toggle.svelte

@@ -1,21 +1,84 @@
 <script>
-  let { checked = $bindable(false), label = "", description = "" } = $props();
+  import { Loader2 } from "lucide-svelte";
+
+  let {
+    checked = $bindable(false),
+    label = "",
+    description = "",
+    /** @type {boolean} disable interaction and dim the toggle */
+    disabled = false,
+    /** @type {boolean} render a spinner in the thumb and block interaction */
+    loading = $bindable(false),
+    /** @type {((next: boolean) => void | Promise<void>) | undefined} optional async handler */
+    onChange = undefined,
+  } = $props();
+
+  // Internal pending flag drives the spinner when an async onChange is in-flight.
+  // Kept separate from the `loading` prop so callers can also force loading externally.
+  let pending = $state(false);
+
+  // Effective busy state covers both the externally-controlled prop and our internal pending flag.
+  let busy = $derived(loading || pending);
+
+  async function handleClick() {
+    if (disabled || busy) return;
+
+    const next = !checked;
+
+    if (onChange) {
+      let result;
+      try {
+        result = onChange(next);
+      } catch (err) {
+        // Synchronous throw: leave checked unchanged, surface the error.
+        throw err;
+      }
+
+      // If the handler returned a thenable, treat it as async and snap back on rejection.
+      if (result && typeof result.then === "function") {
+        pending = true;
+        try {
+          await result;
+          checked = next;
+        } catch (_) {
+          // Swallow; the toggle simply stays at its previous value.
+        } finally {
+          pending = false;
+        }
+      } else {
+        // Synchronous handler: flip immediately after invocation.
+        checked = next;
+      }
+    } else {
+      // No handler provided: preserve original bind:checked behaviour.
+      checked = next;
+    }
+  }
 </script>
 
 <div class="flex items-start gap-3 py-2.5">
   <button
     class="relative mt-0.5 w-[38px] min-w-[38px] h-[22px] rounded-full flex-shrink-0
-      {checked ? 'bg-accent shadow-[0_0_8px_rgba(201,133,85,0.25)]' : 'bg-bg-elevated'}"
-    onclick={() => checked = !checked}
+      {checked ? 'bg-accent shadow-[0_0_8px_rgba(201,133,85,0.25)]' : 'bg-bg-elevated'}
+      {disabled && !busy ? 'opacity-50 cursor-not-allowed' : ''}
+      {busy ? 'cursor-wait' : ''}"
+    onclick={handleClick}
     role="switch"
     aria-checked={checked}
     aria-label={label}
+    aria-disabled={disabled ? "true" : undefined}
+    aria-busy={busy ? "true" : undefined}
   >
     <span
-      class="absolute top-[3px] left-[3px] w-4 h-4 rounded-full bg-white shadow-sm
+      class="absolute top-[3px] left-[3px] w-4 h-4 rounded-full shadow-sm flex items-center justify-center
+        {busy ? '' : 'bg-white'}
         {checked ? 'translate-x-[16px]' : 'translate-x-0'}"
       style="transition: transform var(--duration-ui) cubic-bezier(0.2, 0.8, 0.2, 1)"
-    ></span>
+    >
+      {#if busy}
+        <Loader2 class="w-4 h-4 animate-spin text-accent" />
+      {/if}
+    </span>
   </button>
   <div class="flex-1 min-w-0">
     <p class="text-[13px] text-text leading-tight">{label}</p>