feat(shared-ui): add focusTrap action and apply to shared Modal

Create reusable Svelte action for focus trapping in modals/dialogs.
Traps Tab/Shift+Tab within element and restores focus on destroy.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-ui/src/actions/focusTrap.ts

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+/**
+ * Svelte action that traps focus within an element.
+ * Useful for modals and dialogs to prevent tabbing outside.
+ */
+export function focusTrap(node: HTMLElement) {
+	const focusableSelectors = [
+		'a[href]',
+		'button:not([disabled])',
+		'input:not([disabled])',
+		'select:not([disabled])',
+		'textarea:not([disabled])',
+		'[tabindex]:not([tabindex="-1"])',
+	].join(', ');
+
+	let previouslyFocused: HTMLElement | null = null;
+
+	function getFocusableElements(): HTMLElement[] {
+		return Array.from(node.querySelectorAll<HTMLElement>(focusableSelectors)).filter(
+			(el) => !el.hasAttribute('disabled') && el.offsetParent !== null
+		);
+	}
+
+	function handleKeydown(e: KeyboardEvent) {
+		if (e.key !== 'Tab') return;
+
+		const focusable = getFocusableElements();
+		if (focusable.length === 0) return;
+
+		const first = focusable[0];
+		const last = focusable[focusable.length - 1];
+
+		if (e.shiftKey) {
+			if (document.activeElement === first) {
+				e.preventDefault();
+				last.focus();
+			}
+		} else {
+			if (document.activeElement === last) {
+				e.preventDefault();
+				first.focus();
+			}
+		}
+	}
+
+	// Save currently focused element and focus first focusable in trap
+	previouslyFocused = document.activeElement as HTMLElement;
+
+	// Use requestAnimationFrame to ensure the DOM is ready
+	requestAnimationFrame(() => {
+		const focusable = getFocusableElements();
+		if (focusable.length > 0) {
+			focusable[0].focus();
+		} else {
+			node.focus();
+		}
+	});
+
+	node.addEventListener('keydown', handleKeydown);
+
+	return {
+		destroy() {
+			node.removeEventListener('keydown', handleKeydown);
+			// Restore focus to previously focused element
+			if (previouslyFocused && typeof previouslyFocused.focus === 'function') {
+				previouslyFocused.focus();
+			}
+		},
+	};
+}

packages/shared-ui/src/actions/index.ts

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+export { focusTrap } from './focusTrap';