Disable install button in app install onboarding when no app is selected (#22822)

In the app installation onboarding step, the Install button was always
clickable, even with no app selected. Clicking it with an empty
selection ran the completion flow with zero apps, which is equivalent to
skipping.

Now the button is disabled until at least one app is selected (in
addition to staying disabled while completing). The Skip button remains
available for users who don't want to install anything.

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packages/twenty-front/src/pages/onboarding/InstallAppsContent.tsx

@@ -204,7 +204,9 @@ export const InstallAppsContent = ({
               <MainButton
                 title={t`Install`}
                 onClick={onInstall}
-                disabled={isCompleting}
+                disabled={
+                  isCompleting || !isNonEmptyArray(selectedUniversalIdentifiers)
+                }
                 fullWidth
               />
             </StyledInstallButton>