Show persistent UI indicator when SharedArrayBuffer is unsupported

Problem

We show the SharedArrayBuffer warning in the app, but it is easy for users to circumvent it by clicking around it and accepting the danger. After that, we never show it again — leaving users with no visual indication that they are running in a degraded/unsupported state.

Proposal

Add a persistent visual UI element (with a tooltip on hover) that shows a danger icon for users whose environment does not support SharedArrayBuffer.

A reasonable place for this could be the header, next to the "sync" / "server status" buttons.

The exact visual UI/UX is up for discussion.

Goals

• Make it clear to users that their environment is in an unsupported/dangerous state.
• Persistent indication, not a one-time dismissible warning.
• Provide context via tooltip (what's wrong, why it matters).