feat: avatar by link, loaded by the browser; Files nests workspaces under owners' homes

- POST /api/me/avatar/url stores the https link (users.avatar_link) instead
  of downloading it; GET /api/me/avatar and /api/users/:id/avatar answer 302
  to it (no referrer). Link rules: https, no userinfo, <= 2048 chars, no
  localhost or private/reserved IP literal. Upload and link replace each other.
- Settings -> Account first loads the link as an embedded image and refuses
  one this browser cannot show: a picture behind a login (Mattermost's
  /api/v4/users/<id>/image) gets no cookie cross-site and 401s.
- Files sidebar: each workspace under its owner's home, chevrons remembered
  per browser (agenthub.files.homesOpen); ownerless ones stay flat.

server/routes/users.js

@@ -209,6 +209,12 @@ router.get("/:id", (req, res) => {
 
 router.get("/:id/avatar", (req, res) => {
   const id = Number(req.params.id);
+  if (
+    Number.isSafeInteger(id) &&
+    id > 0 &&
+    require("../lib/avatar-redirect").sendAvatarLink(res, id)
+  )
+    return;
   const file = Number.isSafeInteger(id) && id > 0 ? require("../lib/avatars").avatarFile(id) : null;
   if (!file) return res.status(404).json({ error: { code: "ENOAVATAR", message: "no avatar" } });
   res.set("Content-Type", file.type);