MM-70358: use core's Avatars stack for space members

[calebroseland]

Summary

The +N chip in the space overview member list rendered with a see-through background, letting the
avatar underneath show through it. The cause wasn't the chip: Docs had reimplemented core's whole
avatar stack, because core only published Avatar to plugins and not Avatars.

The copy had drifted from core in eight ways — translucent chip fill
(rgba(--center-channel-color-rgb, 0.08) where core computes an opaque
tinycolor.mix(bg, color, 8)), a hardcoded -8px offset and 2px ring instead of core's per-size
--offset / --border-width, a bare <span> chip rather than an Avatar, no profile popover, no
keyboard access, no hover affordance, no 99+ cap or totalUsers handling, and a different
break point.

So rather than patch the chip and leave the other seven, this deletes the reimplementation and
renders core's Avatars.

Host compatibility. Avatars is published by mattermost#38165, which hasn't landed. Docs must
keep running against hosts that predate it, so member_avatars.tsx probes hostHasAvatars() and
falls back to the previous local stack — with the reported bug fixed there too: the chip fill is
now opaque via color-mix, and the offset and ring match core at sm. Once the host publishes
Avatars the fallback stops being reachable, and the overflow chip also becomes clickable
(mattermost#38166) via canOpenOverflow.

Testing

• Unit: both paths covered — host-publishes (ids forwarded, canOpenOverflow set, no fallback) and
  host-doesn't (fallback renders, overflow counting). 464 tests pass.
• Playwright specs added for the space member avatars. They skip when the host doesn't publish
  Avatars, since they assert core's counts and popover behaviour. They have not been executed
  against a live server — no available host publishes Avatars yet.

Ticket Link

Fixes: https://mattermost.atlassian.net/browse/MM-70358

Depends on

• MM-70358: export Avatars on window.Components mattermost#38165 — exports Avatars (required for the non-fallback path)
• feat(avatars): clickable +N overflow opens a list of those users mattermost#38166 — adds canOpenOverflow (stacked on the above)