Validate mobile sheet in browser (hamburger → sheet < md)

During the web-shell plan implementation, the mobile sheet open/close was validated via jsdom tests (keyboard Escape key, aria attributes). Browser validation via chrome-devtools-axi was not possible at < md viewport width.

**What to check:**
- At mobile viewport (< 768px), header collapses to: logo + hamburger + auth control
- Tapping hamburger opens the Sheet from the right with stacked nav links
- Sheet contains: Projects, Help Wanted, Members, Volunteer, separator, About section, search box
- Focus is trapped while sheet is open
- Sheet closes on Escape or overlay click and returns focus to the hamburger trigger

**When to close:** Add to the QA checklist for `public-screens` PR review.

_From: plans/web-shell.md Follow-ups_

Start with the mobile sheet follow-up in plans/web-shell.md and the public-screens PR review context. Run the browser checks with chrome-devtools-axi below 768px, covering opening, navigation, focus trapping, and close behavior. Done means the validated checks are added to the QA checklist.

Written by the indexing model from the issue text.