Pause hidden split-pane timeline queries

[bb-slop-cop]

Human comments

What was wrong

Maximizing a split pane leaves sibling thread views mounted with enabled timeline query observers. An invalidation can refetch a sibling even while its pane is hidden. The direct reproduction and root-cause evidence are in the report.

What changed

The split layout passes pane visibility to the thread detail view. Hidden siblings disable their timeline observer and resume fetching when shown again. The pane and its local state stay mounted. No wire, CLI, or stored-data contract changed.

How you verified

• The focused query-observer regression failed on trusted origin/main in two clean checkouts: expected one hidden-pane fetch, observed two.
• The regression passed after the fix. The related split-pane, timeline-controller, and timeline-query suites passed: 101 tests total.
• pnpm exec turbo run lint typecheck --filter=@bb/app passed; lint reported 209 existing warnings and no errors.
• git diff --check passed. The patch changes 78 text lines (70 additions, 8 deletions) across three files and includes no binary changes.

Fixes #4508

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