Overlays don't trap focus #50

Summary

With a Dialog, Sheet, Drawer or AlertDialog open, Tab moves focus to elements behind the overlay. Keyboard and screen-reader users can end up in content they can't see.

Steps to reproduce

1. Open any Dialog.
2. Press Tab repeatedly.

Expected: focus cycles through the dialog's focusable elements only, and returns to the trigger when the dialog closes.
Actual: focus leaves the dialog and reaches the page behind it.

Suggested fix

A small focus-trap helper in shellui.js (package and CLI template), called when an overlay opens and released when it closes: focus the first focusable element (or the content), wrap Tab / Shift+Tab, restore focus to the trigger on close. Apply it to Dialog, AlertDialog, Sheet and Drawer, and add tests.