Update env docs for Airstack

Add Airstack follower graph option

Update env template for Hub backend

Remove Neynar; use Hub for follower graph

Add Hub-based follower graph (no Neynar required)

Fix miniapp splash and Neynar key handling

Use logo on splash

Adjust splash and improve Neynar auth

Restore splash icon

Simplify splash and improve graph errors

Show detailed graph API errors

Fallback to public graph on auth failure

Verify Quick Auth using token audience

Use Quick Auth fetch for graph

Support web mode and fix Quick Auth domain