fix(dashboard): F-U-016 focus trap in ConfirmModal (Tab/Shift+Tab cycle)

ConfirmModal focused the first input on mount and listened for Escape,
but didn't trap Tab. Keyboard users could Tab out of the modal into
the underlying page — a strong accessibility violation and a confusing
keyboard-UX moment.

Add a handleTrapTab keydown handler on the dialog div that:
- Enumerates focusables (button:not([disabled]), input:not([disabled]),
  select, textarea, a[href], [tabindex]:not(-1)).
- On Tab from the last focusable, wraps to the first.
- On Shift+Tab from the first, wraps to the last.

The dialog ref is bound via bind:this; the handler is no-op if the
dialog isn't mounted yet.

AUDIT.md anchor: F-U-016.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/src/lib/ConfirmModal.svelte

@@ -90,11 +90,34 @@
 	// doesn't auto-confirm a destructive action).
 	let inputRef = $state<HTMLInputElement | null>(null);
 	let cancelRef = $state<HTMLButtonElement | null>(null);
+	let dialogRef = $state<HTMLDivElement | null>(null);
 
 	$effect(() => {
 		if (inputRef) inputRef.focus();
 		else if (cancelRef) cancelRef.focus();
 	});
+
+	// F-U-016: focus trap. The browser will otherwise Tab into the
+	// underlying page; for a modal that's a strong accessibility and
+	// keyboard-UX violation. Cycle focus within the dialog and reverse
+	// on Shift+Tab.
+	function handleTrapTab(event: KeyboardEvent) {
+		if (event.key !== 'Tab' || !dialogRef) return;
+		const focusables = dialogRef.querySelectorAll<HTMLElement>(
+			'button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), a[href], [tabindex]:not([tabindex="-1"])'
+		);
+		if (focusables.length === 0) return;
+		const first = focusables[0]!;
+		const last = focusables[focusables.length - 1]!;
+		const active = document.activeElement as HTMLElement | null;
+		if (event.shiftKey && active === first) {
+			event.preventDefault();
+			last.focus();
+		} else if (!event.shiftKey && active === last) {
+			event.preventDefault();
+			first.focus();
+		}
+	}
 </script>
 
 <svelte:window onkeydown={handleKeydown} />
@@ -105,11 +128,13 @@
 	onclick={handleBackdrop}
 >
 	<div
+		bind:this={dialogRef}
 		class="dialog"
 		class:danger={variant === 'danger'}
 		role="dialog"
 		aria-modal="true"
 		aria-labelledby="confirm-title"
+		onkeydown={handleTrapTab}
 	>
 		<h2 id="confirm-title">{title}</h2>
 		<div class="body">