Each stat card now shows the percentage change vs the equivalent
previous period (e.g. 30d compares last 30 days vs 30 days before).
Handles zero-baseline with "new" label and caps extreme deltas at
>999%. Seed data extended to 2 years for meaningful 12m comparisons.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>