🎇 Add strictNullChecks to the tsconfig

[ansballard]

Currently we ignore null | undefined returns in useKV, even though the types explicitly list T | undefined as part of the return. This change surfaces that possible undefined to the user so it can be resolved by them or the agent in a visible way.

[Copilot]

Copilot wasn't able to review any files in this pull request.

[ansballard]

Confirmed in several fresh sparks that if you do nothing but add strictNullChecks to the tsconfig, that iterations and publishes both still work as expected. Confirmed that there can be typescript exceptions in the app and builds still succeed, because we pass --noChecks to the build command, and there is a typescript error in src/components/ui/calendar.tsx that would block any builds in sparks that use calendars. So this should be pretty safe 🚀