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/__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 () => {