Recover referral invite form after send failures

[morganschp]

Summary

• recover the referral invite form after network failures or non-JSON error responses
• add Clipboard API fallback copying for the referral link button
• show a manual-copy error when both clipboard paths fail
• add component regression coverage for send failures and copy fallback/failure paths

Fixes #131
Fixes #129

Validation

• ./node_modules/.bin/vitest run src/app/dashboard/referrals/page.test.tsx
• ./node_modules/.bin/eslint src/app/dashboard/referrals/page.tsx src/app/dashboard/referrals/page.test.tsx
• ./node_modules/.bin/prettier --check src/app/dashboard/referrals/page.tsx src/app/dashboard/referrals/page.test.tsx
• ./node_modules/.bin/tsc --noEmit --pretty false
• git diff --check -- src/app/dashboard/referrals/page.tsx src/app/dashboard/referrals/page.test.tsx

uGig gig: https://ugig.net/gig/cmexbmp260001ju04n3qmkdko