Consistency pass across the App Center Overview area: no tab has a top-level
manual "Check" button anymore — scans are automatic.
- Overview tab: remove the header "Check now"; the hero already reports last-scan
time, and the unscanned sub-copy no longer points at a button that's gone.
- Improvements tab: remove the header "Check"; lead its body with the same
auto-check line (right-side Check-now nudge) as the Updates tab.
- Remove the now-unused checkBtn helper.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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