fix(expo): resolve packId type incompatibility in TripForm

[Copilot]

TypeScript compilation failed because packId in the form schema allows null but the Trip and TripInput types only accept string | undefined.

Changes

• Convert null packId to undefined before passing to API functions in the submit handler

const submitData = {
  ...value,
  location: location ?? value.location,
  packId: value.packId ?? undefined,  // null → undefined
};

This maintains the form schema's ability to represent "no pack selected" as null while ensuring type compatibility with the API layer.

[Isthisanmol]

Looks good to me.
I should’ve considered doing this too.