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/lib/avatar-redirect.js

@@ -0,0 +1,21 @@
+/**
+ * @file avatar-redirect.js
+ * @description Answers an avatar GET (`/api/me/avatar`, `/api/users/:id/avatar`)
+ * with a 302 to the account's stored avatar LINK (server/lib/avatars.js), so
+ * the viewer's browser loads the picture itself — with its own logins and
+ * network — and the server never fetches it. No referrer is sent with it.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+/** True when the account has a link and the redirect was sent. */
+function sendAvatarLink(res, userId) {
+  const link = require("./avatars").avatarLink(userId);
+  if (!link) return false;
+  res.set("Referrer-Policy", "no-referrer");
+  // The client versions the URL with `avatarAt`, so a new link is a new URL.
+  res.set("Cache-Control", "private, max-age=86400");
+  res.redirect(302, link);
+  return true;
+}
+
+module.exports = { sendAvatarLink };