Keep keyboard focus inside the open popup chat #335

When the popup chat is open, Tab and Shift+Tab can leave the panel and land on the host page behind it. The panel is a dialog (role="dialog", Escape closes it, focus returns to the launcher), so focus should cycle inside it until it's closed (WCAG 2.4.3, focus order).

What to do

• In popup mode, while the panel is open, Tab from the last focusable element goes to the first, and Shift+Tab from the first goes to the last.
• Work out the focusable elements when Tab is pressed, not once at open. The pre-chat form, retry and regenerate buttons come and go.
• Inline mode is part of the page, so leave it alone.
• Add aria-modal="true" to the popup panel next to role="dialog".

Where to look

• packages/widget/src/dom/panel.ts: the constructor sets role="dialog" and handles Escape in a keydown listener. The trap can go in the same listener.
• The panel lives in a shadow root, so the focused element is shadowRoot.activeElement, not document.activeElement (see containsFocus()).
• packages/widget/tests/dom/panel.test.ts

Acceptance criteria

• Tab and Shift+Tab wrap inside the open popup, including while the pre-chat form is shown
• Inline mode and a closed popup are unaffected
• aria-modal="true" on the popup panel
• Unit tests for both directions and for inline mode
• Changeset for @chatcops/widget (patch)
• pnpm test:coverage, pnpm lint and pnpm size pass

The chat panel (dist/chatcops.panel.js) has a 14 kB gzip budget and about 800 B of it is left, shared by several open issues. Keep the change small, and check pnpm size before you open the PR.