Fix conversation menu clicks blocked by the header layout

[wangtaotaotao95]

Closes #122.

The centered identity wrapper spans the header and is painted over the conversation menu button, so its transparent area intercepts ordinary pointer clicks. Set pointerEvents="box-none" on that wrapper: empty space lets clicks reach the menu while the identity/activity and computer controls remain interactive.

Validation on Node 24.21.0 / pnpm 11.19.0:

• Chromium reproduced blocked, non-forced menu clicks before the fix at 1280×1000 and 390×844. After the fix, ordinary clicks open the menu at both sizes; the identity/activity and notification buttons also remain clickable.
• All 276 tests, lint, root/mobile/worker type checks, server build, and Web/iOS/Android exports pass.
• Used fictional sample data with stubbed Rich Threads provisioning. Native simulator/device runtime and live providers were not tested; phone screenshots are Web previews.

AI assistance: investigated, implemented and verified with Codex.