feat: auto-detect available time ranges in chart + video exporter

[Flotapponnier]

Summary

• Chart probes 90d after 30d data is confirmed, then probes 1y after 90d — tabs appear dynamically only when data exists
• 7d/30d tabs are hidden entirely when confirmed empty (not just disabled)
• Auto-fallback: if the active range has no data, switches to the nearest available range
• Video exporter probes all ranges on modal open when not wired to the chart; shows loading shimmer while probing, auto-selects the best available range
• onAvailableRangesChange callback and availableRanges prop allow optional chart→exporter state sharing

Test plan

• Open aggregator-head-lag (has 30d data) — 90d tab should appear after a moment, then 1y
• Open a bench with no 30d data — 90d/1y tabs should never appear
• Open the video export modal — range selector shows shimmer then filters to available ranges
• Switch to a range with no data — chart auto-falls back to 24h