fix(web): preserve Done status for threads left mid-turn

[asfires]

What Changed

• Seed the active thread's visited timestamp from its creation, request, or start time before a turn completes.
• Preserve the completed timestamp as unread when the user leaves a thread during an in-flight turn, while clearing Done normally when the thread remains open through completion.
• Simplify typography settings by removing the configurable tool-output font size and using a fixed 11px size.
• Remove obsolete provider resume-cursor persistence and stale theme/font tests and settings plumbing.

Why

Threads opened while a turn was still running could lose the later completion signal because the visit marker was not established until completion. The new baseline ensures a thread left mid-turn correctly appears as Done when the turn finishes, without changing the behavior for users who keep the thread open.

The typography and provider cleanup removes unused configuration and synchronization paths that are no longer needed.

UI Changes

• Removed the Tool output font size setting from Appearance settings.
• Tool output now uses a consistent 11px font size.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

[asfires]

Closing in favor of replacing the narrow active-thread baseline fix with an adaptation of upstream issue pingdotgg#3131's live-completion tracker (commit 5014079).