fix(settings): require integer retention days

[aiirvizionz]

Summary

• reject fractional disappearing-message retention values
• keep retention settings limited to non-negative whole days
• add regression coverage that prevents decimal values from reaching the update query

Test

• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs "src/app/api/settings/disappearing-messages/route.test.js"
• git diff --check

uGig billing

• Expected invoice: 0.50 USD if accepted/merged
• Counted as revenue now: no, pending review/payment