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/openapi-extra/auth.js

@@ -14,7 +14,7 @@
  *   - GET/PATCH /api/me/account, POST /api/me/password, POST /api/me/sign-out-others
  *     (Auth) — Settings → Account: the account's own profile, password and logins
  *   - GET/PUT/DELETE /api/me/avatar (Auth) — the account's own picture
- *   - POST /api/me/avatar/url (Auth) — the same picture downloaded from a link
+ *   - POST /api/me/avatar/url (Auth) — the picture as a link, redirected to
  *   - GET/PATCH /api/users/{id} (Auth) — the Users page's Edit user form
  *   - GET /api/users/{id}/avatar (Auth) — one account's picture (admin/analyst)
  *   - GET /api/users/tmux, POST /api/users/{id}/tmux-restart, POST /api/users/tmux-restart-idle
@@ -309,9 +309,10 @@ const paths = {
     get: {
       tags: ["Auth"],
       operationId: "getMyAvatar",
-      summary: "The account's own picture (nosniff, sandbox CSP)",
+      summary: "The account's own picture (nosniff, sandbox CSP), or a 302 to its avatar link",
       responses: {
         200: { description: "The image (PNG, JPEG, GIF or WebP)" },
+        302: { description: "Redirect to the account's avatar link" },
         401: err("EAUTH"),
         404: err("ENOAVATAR"),
       },
@@ -500,7 +501,7 @@ const paths = {
       tags: ["Auth"],
       operationId: "setMyAvatarFromUrl",
       summary:
-        "Set the account's picture from an https link the server downloads (public addresses only, at most 3 redirects, PNG/JPEG/GIF/WebP, at most 1 MB). Login cookie only, audited",
+        "Set the account's picture to an https link, stored and never downloaded — the avatar GETs answer 302 to it (no userinfo, no localhost/private IP literal, at most 2048 chars). Login cookie only, audited",
       requestBody: {
         required: true,
         content: {
@@ -515,11 +516,9 @@ const paths = {
       },
       responses: {
         200: { description: "Set", content: accountBody },
-        400: err("EBADURL / EPRIVATEURL / EBADIMAGE"),
+        400: err("EBADURL / EPRIVATEURL"),
         401: err("EAUTH — no login cookie (a pane token is refused)"),
         403: err("ELOCALMODE"),
-        413: err("ETOOBIG"),
-        502: err("EFETCH — the link could not be downloaded"),
       },
     },
   },
@@ -531,6 +530,7 @@ const paths = {
       parameters: [{ name: "id", in: "path", required: true, schema: { type: "integer" } }],
       responses: {
         200: { description: "The image (PNG, JPEG, GIF or WebP)" },
+        302: { description: "Redirect to that account's avatar link" },
         403: err("EADMIN — a developer"),
         404: err("ENOAVATAR"),
       },