resolve_quota only ever reported the caller's personal envelope,
ignoring the drive_id already resolved at every PROPFIND call site —
shared drives with their own quota showed the wrong numbers. Adds
AppState::resolve_webdav_quota, shared by both WebDAV surfaces:
nil drive_id or personal drive -> account envelope, shared drive ->
its own storage.drives quota/used_bytes.
Also adds quota-used-bytes/quota-available-bytes to the NextCloud-
compatible surface, which previously had no RFC 4331 support at all.
Registers webdav_quota_properties.hurl and the new
nc_webdav_quota_properties.hurl in tests/api/run.sh — neither was
wired into the suite before this change.
M.Schmidt