Audit avatar sizing assumptions

See nextcloud/files-clients#18

Since v24.0.0 the Server Avatar API only returns 64 or 512.

When other sizes are requested, the server API "rounds" to the nearest match unbeknownst to the requesting client.

Concerns:

- When the client retrieves the wrong size, caching is impacted (due to mismatched URLs)
- When the client receives the wrong size, there may be display or other cosmetic issues.

Needs:

- Only 64 or 512 should be retrieved to avoid breaking caching.
- Any assumptions about sizes other than 64/512 being available should be handled.

References:

- nextcloud/server#31010
- nextcloud/server#47211