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/avatars.js

@@ -5,7 +5,9 @@
  * `<data dir>/avatars/u<id>.<png|jpg|gif|webp>` — a name made from the
  * account id and the type its magic bytes say, never from the request — and
  * `users.avatar_at` records when it was set, so the client can version the
- * URL. Raster only: an SVG served from the dashboard's origin runs script.
+ * URL. Or, instead of a file, a LINK (`users.avatar_link`, checked by
+ * avatar-url.js) the serving routes redirect the browser to — never
+ * downloaded. Setting one clears the other. Raster only: an SVG served from the dashboard's origin runs script.
  * At most `MAX_AVATAR_BYTES`; the serving route adds `nosniff` and a
  * `sandbox` CSP like every other user-supplied file.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
@@ -32,10 +34,35 @@ function fileName(userId, ext) {
 
 const db = () => require("../db").db;
 
-/** Deletes every stored avatar file of the account and clears `avatar_at`. */
-function removeAvatar(userId) {
+const removeFiles = (userId) => {
   for (const ext of EXTS) fs.rmSync(fileName(userId, ext), { force: true });
-  db().prepare("UPDATE users SET avatar_at = NULL WHERE id = ?").run(Number(userId));
+};
+
+/** Deletes every stored avatar file of the account and clears `avatar_at` and the link. */
+function removeAvatar(userId) {
+  removeFiles(userId);
+  db()
+    .prepare("UPDATE users SET avatar_at = NULL, avatar_link = NULL WHERE id = ?")
+    .run(Number(userId));
+}
+
+/** Stores `text` (EBADURL / EPRIVATEURL) as the account's avatar link; returns `avatar_at`. */
+function setAvatarLink(userId, text) {
+  const link = require("./avatar-url").parseAvatarUrl(text);
+  removeFiles(userId);
+  const at = new Date().toISOString();
+  db()
+    .prepare("UPDATE users SET avatar_at = ?, avatar_link = ? WHERE id = ?")
+    .run(at, link, Number(userId));
+  return at;
+}
+
+/** The account's avatar link, or null when it has none. */
+function avatarLink(userId) {
+  return (
+    db().prepare("SELECT avatar_link FROM users WHERE id = ?").get(Number(userId))?.avatar_link ||
+    null
+  );
 }
 
 /** Stores `buf` as the account's avatar (EBADIMAGE / ETOOBIG); returns `avatar_at`. */
@@ -54,7 +81,9 @@ function setAvatar(userId, buf) {
     if (other !== ext) fs.rmSync(fileName(userId, other), { force: true });
   }
   const at = new Date().toISOString();
-  db().prepare("UPDATE users SET avatar_at = ? WHERE id = ?").run(at, Number(userId));
+  db()
+    .prepare("UPDATE users SET avatar_at = ?, avatar_link = NULL WHERE id = ?")
+    .run(at, Number(userId));
   return at;
 }
 
@@ -68,4 +97,12 @@ function avatarFile(userId) {
   return null;
 }
 
-module.exports = { MAX_AVATAR_BYTES, setAvatar, removeAvatar, avatarFile, avatarsDir };
+module.exports = {
+  MAX_AVATAR_BYTES,
+  setAvatar,
+  setAvatarLink,
+  removeAvatar,
+  avatarFile,
+  avatarLink,
+  avatarsDir,
+};