perf(user_status): skip heartbeats another tab already sent

[pringelmann]

Summary

Every tab and every page load announced presence on its own, so the dashboard sent two heartbeats (it mounts two instances) and hopping between apps sent one per page.

They now share a lastHeartbeat timestamp in browser storage and send at most one announcement per four minutes, well inside the server's fifteen minute staleness threshold. A tab opened in the background waits until it's actually looked at.

The five minute interval is longer than the four minute window, so a single tab keeps its existing cadence and the gap to the server never grows. Coming back from away is never throttled.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes: no visual change
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI (tests, reviewed)

[pringelmann]

/backport to stable35