share-card modal: scope dropdowns for chain/region/venue/kind

[Flotapponnier]

Summary

• Adds a scope picker row inside the export-as-image modal so readers can override chain / region / venue / kind without leaving the modal
• Only dimensions the bench actually declares get a dropdown (chain-less benches never see a chain dropdown)
• Pings /api/stat/? on each pick and warns the reader when the chosen cell has no data (endpoint returns 404 on unknown filter combos, exactly the signal we need)
• Modal opens pre-filled from window.location so the dropdowns match what the reader was filtering on the dashboard; values that vanished between deploys coerce back to "All" instead of a broken picker

Test plan

• Open the export modal on a bench with dimensions (e.g. /benchmarks/rpc-latency): dropdowns render for chain + region
• Open it on a bench without dimensions (e.g. /benchmarks/bridge-revenue equivalent): no dropdowns render
• Pick an empty combo: amber "No data for this scope" hint appears; PNG still generates (server falls back to aggregate)
• Pick a valid combo: preview refreshes, PNG downloads with the right pill label