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/__tests__/me-avatar.test.js

@@ -7,8 +7,9 @@
  * another account never sees it through `/api/me`; staff read it at
  * `/api/users/:id/avatar` (a developer is refused) and a workspace carries
  * `owner_avatar_at` for the sidebar's owner groups. A picture from a link
- * (`POST /api/me/avatar/url`, server/lib/avatar-url.js) takes https only and
- * never reaches a loopback/private address.
+ * (`POST /api/me/avatar/url`, server/lib/avatar-url.js) is stored, never
+ * downloaded, and the avatar routes redirect to it; https only, never a
+ * loopback/private IP literal.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -110,6 +111,46 @@ it("refuses an SVG, an empty body, a too-large image and no login", async () =>
   assert.equal((await put("", PNG)).status, 401);
 });
 
+it("a picture from a link is stored and redirected to, never downloaded", async () => {
+  const ava = await login("ava", "ava-pass-1");
+  const ann = await login("ann", "ann-pass-1");
+  const link =
+    "https://chat.runsystem.vn/api/v4/users/7bya4wy58bdk3qmb5ks1rgoqkh/image?_=1776312011651";
+  assert.equal((await put(ava, PNG)).status, 200);
+  const set = await fetch(BASE + "/api/me/avatar/url", {
+    method: "POST",
+    headers: { cookie: ava, "Content-Type": "application/json" },
+    body: JSON.stringify({ url: ` ${link} ` }),
+  });
+  assert.equal(set.status, 200);
+  assert.ok((await set.json()).user.avatarAt);
+  // The uploaded file is gone: a link replaces it.
+  assert.equal(fs.readdirSync(path.join(TMP, "avatars")).length, 0);
+
+  const got = await fetch(BASE + "/api/me/avatar", {
+    headers: { cookie: ava },
+    redirect: "manual",
+  });
+  assert.equal(got.status, 302);
+  assert.equal(got.headers.get("location"), link);
+  assert.equal(got.headers.get("referrer-policy"), "no-referrer");
+  const id = auth.findUser("ava").id;
+  const staff = await fetch(`${BASE}/api/users/${id}/avatar`, {
+    headers: { cookie: ann },
+    redirect: "manual",
+  });
+  assert.equal(staff.status, 302);
+  assert.equal(staff.headers.get("location"), link);
+
+  // An upload replaces the link.
+  assert.equal((await put(ava, PNG)).status, 200);
+  assert.equal(
+    (await fetch(BASE + "/api/me/avatar", { headers: { cookie: ava }, redirect: "manual" })).status,
+    200
+  );
+  await fetch(BASE + "/api/me/avatar", { method: "DELETE", headers: { cookie: ava } });
+});
+
 it("a picture from a link: https only, never a private or local address", async () => {
   const ava = await login("ava", "ava-pass-1");
   const post = (url) =>