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_