fix(settings): read social sync initial state as boolean

[MiMoHo]

Summary

Since babe45a the PageController provides allowSocialSync and enableSocialSync as booleans (SocialApiService::syncAllowedByAdmin() / backgroundSyncEnabled() return bool), but ContactsSettings.vue still compared the initial state with the strings 'yes'/'no':

allowSocialSync: loadState('contacts', 'allowSocialSync') !== 'no',
enableSocialSync: loadState('contacts', 'enableSocialSync') !== 'no',

A boolean is never === 'no', so both flags were always true: the Update avatars from social media switch always appeared enabled, regardless of the stored setting.

Changes

• Read both states as booleans, with fallbacks matching the server defaults (allowSocialSync: true, enableSocialSync: false).
• allowSocialSync was loaded into data() but no longer used anywhere since the settings dialog refactorings (it used to guard the switch, see 973c288 / 7310a2a). Put it back to work: the switch is now hidden when the administrator has disabled social sync, instead of showing users a toggle that has no effect (backgroundSyncEnabled() is false whenever syncAllowedByAdmin() is).

Note on AdminSettings.vue

The issue also suggests changing AdminSettings.vue, but that would break it: its initial state comes from lib/Settings/AdminSettings.php, which provides the raw string app values ('yes'/'no') from Application::AVAIL_SETTINGS, so the === 'yes' comparison is correct there. Left unchanged on purpose.

Checklist

• Jest suite passes, npm run lint clean for the touched file (0 errors), production build succeeds
• Manually traced both initial-state providers (PageController → booleans, Settings/AdminSettings.php → strings)

Resolve #5561

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