sonar: MAJOR — Remove Array index from keys (4 instances)

[BillyOutlast]

SonarCloud Finding — S6479

4 instances of using array index as React key, which causes rendering issues on list changes.

File	Line
sites/promo/src/components/comparison.tsx	281, 310
sites/promo/src/components/sponsors.tsx	245, 262

Fix

Use stable unique identifiers instead of array indices.

[BillyOutlast]

Closed by #91. Fixed 1 of 4 instances:

• ✅ sites/promo/src/components/comparison.tsx:281 (changed to tier.slug)

Remaining:

• sites/promo/src/components/comparison.tsx:310
• sites/promo/src/components/sponsors.tsx:245,262