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.

client/src/components/settings/AccountSettings.tsx

@@ -4,7 +4,7 @@
  * account's own information (`GET /api/me/account` — username, role, member
  * since, workspaces, sessions, active sign-ins, last sign-in), its avatar
  * (`PUT`/`DELETE /api/me/avatar` — PNG, JPEG, GIF or WebP, at most 1 MB — or
- * an https link the server downloads, `POST /api/me/avatar/url`; the sidebar
+ * an https link the browser loads itself, `POST /api/me/avatar/url`; the sidebar
  * shows it at once), its display name
  * and email (`PATCH /api/me/account`), a password change that asks for the
  * current one and passes every rule of `lib/passwordRules.ts`, ticked as it is
@@ -48,6 +48,24 @@ const errText = (t: (k: string, o?: Record<string, unknown>) => string, e: unkno
     : err.message;
 };
 
+/** Whether `url` loads as an image embedded in this page — the way every
+ *  avatar is shown: no referrer, the cookies a cross-site image gets. */
+export function loadsAsImage(url: string, timeoutMs = 10_000): Promise<boolean> {
+  return new Promise((resolve) => {
+    const img = new Image();
+    const done = (ok: boolean) => {
+      window.clearTimeout(timer);
+      img.onload = img.onerror = null;
+      resolve(ok);
+    };
+    const timer = window.setTimeout(() => done(false), timeoutMs);
+    img.referrerPolicy = "no-referrer";
+    img.onload = () => done(img.naturalWidth > 0);
+    img.onerror = () => done(false);
+    img.src = url;
+  });
+}
+
 export function AccountSettings() {
   const { t } = useTranslation("settings");
   const [account, setAccount] = useState<ServerAccount | null>(null);
@@ -138,6 +156,14 @@ function ProfileCard({
   const applyLink = async () => {
     const url = link?.trim();
     if (!url) return;
+    // The avatar routes only redirect to the link, so it shows only if THIS
+    // browser can load it as an embedded image: a link behind a login (a
+    // Mattermost /api/v4/users/<id>/image) gets no cookie from another site
+    // and fails — say so now instead of saving a picture nobody sees.
+    setBusy(true);
+    const ok = await loadsAsImage(url);
+    setBusy(false);
+    if (!ok) return void toast.error(t("account.avatar.linkUnreachable"));
     if (await run(() => api.me.setAvatarFromUrl(url), t("account.avatar.saved"))) setLink(null);
   };
   const pick = (e: ChangeEvent<HTMLInputElement>) => {

client/src/components/settings/__tests__/AccountSettings.test.tsx

@@ -78,8 +78,40 @@ describe("AccountSettings", () => {
     expect(await screen.findByTestId("user-avatar-initials")).toBeTruthy();
   });
 
+  /** jsdom never loads an image: play the browser's answer to `src`. */
+  const stubImage = (loads: boolean) =>
+    vi.stubGlobal(
+      "Image",
+      class {
+        naturalWidth = loads ? 64 : 0;
+        referrerPolicy = "";
+        onload: (() => void) | null = null;
+        onerror: (() => void) | null = null;
+        set src(_v: string) {
+          setTimeout(() => (loads ? this.onload?.() : this.onerror?.()), 0);
+        }
+      }
+    );
+
+  it("refuses a link this browser cannot load as an image, and saves nothing", async () => {
+    const ue = userEvent.setup();
+    stubImage(false);
+    render(<AccountSettings />);
+    await screen.findByTestId("account-card");
+    await ue.click(screen.getByTestId("account-avatar-url-toggle"));
+    await ue.type(
+      screen.getByTestId("account-avatar-url-input"),
+      "https://chat.example/api/v4/users/x/image{Enter}"
+    );
+    await waitFor(() => expect(screen.getByTestId("account-avatar-url-input")).toBeTruthy());
+    await new Promise((r) => setTimeout(r, 20));
+    expect(me.setAvatarFromUrl).not.toHaveBeenCalled();
+    vi.unstubAllGlobals();
+  });
+
   it("sets the avatar from a link and closes the field", async () => {
     const ue = userEvent.setup();
+    stubImage(true);
     const withAvatar = { ...account, user: { ...account.user, avatarAt: "2026-10-21T00:00:00Z" } };
     me.setAvatarFromUrl.mockResolvedValue(withAvatar);
     render(<AccountSettings />);
@@ -92,6 +124,7 @@ describe("AccountSettings", () => {
       "/api/me/avatar?v=2026-10-21T00:00:00Z"
     );
     expect(screen.queryByTestId("account-avatar-url-input")).toBeNull();
+    vi.unstubAllGlobals();
   });
 
   it("shows the account's own information", async () => {