feat(poll): enables background refresh when tab is hidden

[wgordon17]

Summary

• Removes visibility guard from full poll so it continues fetching in background tabs — but only when the notifications gate is available (304 responses make this near-zero cost)
• When notifications gate is disabled (fine-grained PAT or missing scope), background polling pauses to conserve API budget; catch-up fetch on tab return ensures data freshness
• Retains visibility guard in hot poll (30s) — visual-only feedback has no value in hidden tabs
• Retains visibilitychange catch-up handler as safety net for browser tab throttling (Safari purge, Chrome Energy Saver)