fix(lib): null check ordering in invite validation flow

[Copilot]

The invite validation logic checked invitingProfile for null after using invitingProfileId in subsequent validations, creating a potential null reference path if itemsApi.getItem() returned null.

Changes

• Moved invitingProfile null check immediately after fetch, before self-invite and already-following checks

Before

const invitingProfile = await itemsApi.getItem(invitingProfileId)

if (invitingProfileId === myProfile?.id) { ... }  // Uses invitingProfileId
if (myProfile?.relations?.some(...)) { ... }      // Uses invitingProfileId
if (!invitingProfile) { ... }                     // Null check comes last

After

const invitingProfile = await itemsApi.getItem(invitingProfileId)

if (!invitingProfile) { ... }                     // Null check first
if (invitingProfileId === myProfile?.id) { ... }
if (myProfile?.relations?.some(...)) { ... }

This ensures the profile exists before downstream validation logic executes.

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[Copilot]

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