feat: add duplicate dashboard with all cards

[aalvaropc]

Summary

• "Duplicate" button on dashboard show page toolbar
• New duplicate_dashboard/2 in Dashboards context that clones dashboard + all cards in a transaction
• Cloned dashboard gets title with " (copy)" suffix
• Cards preserve type, config (span, filter_field, prefix/suffix), position, and saved_query_id
• Navigates to the new dashboard after duplication

Test plan

• mix precommit — 638 tests, 0 failures
• Manual: click Duplicate on a dashboard with cards, verify new dashboard has all cards with same config