Fix NIGHTLY passive update pill discovery
Fixes https://github.com/manaflow-ai/cmux/issues/12467
The passive Sparkle update probe records availability in UpdateStateModel, but the sidebar footer is inside an Equatable sidebar subtree that only carries the model reference. In-place passive mutations can therefore be skipped until an unrelated redraw. This PR adds a delayed passive-callback UI regression that runs after the sidebar mounts; the first commit intentionally fails on the current implementation so CI proves the repro.
Validation so far: git diff --check.
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Summary by cubic
Fixes passive Sparkle update discovery so a detected update that arrives after the sidebar mounts still shows in the update pill. Adds a regression UI test that simulates a delayed passive update after the sidebar mounts.