Show almost-instant preview when opening lightbox (#6000)

* Plumb thumbUri down to the lightbox

* Remove onLoad tracking from lightbox

* Hook up placeholder URI to the image

* Fix NaN causing crash on double tap while offline

* Protect against NaNs in the future

src/view/com/profile/ProfileSubpageHeader.tsx

@@ -72,7 +72,7 @@ export function ProfileSubpageHeader({
     ) {
       openLightbox({
         type: 'images',
-        images: [{uri: avatar}],
+        images: [{uri: avatar, thumbUri: avatar}],
         index: 0,
       })
     }