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.

docs/SERVER-MODE.md

@@ -345,7 +345,10 @@ remembered, so the default is always the admin's own account.
 
 Every signed-in user has **Settings → Account**: their username, role, member
 since, workspace and session counts and active logins; their own picture
-(PNG, JPEG, GIF or WebP up to 1 MB, kept in `<data dir>/avatars/`; shown with
+(PNG, JPEG, GIF or WebP up to 1 MB, kept in `<data dir>/avatars/`, or an
+https link to one, which the browser loads itself — never downloaded, so a
+link behind a login (a Mattermost `/api/v4/users/<id>/image`) cannot show and
+is refused when saved; shown with
 their name and role at the foot of the sidebar, which opens this page); their
 own display name and email; **Change password** (the current password is required, the login
 it is changed from stays and every other one ends); and **Sign out other