feat(cabana): add backend user avatars to My account and the top bar

- cabana.BackendUser is an attach.Owner (MorphName Backend\Models\User,
  cabana.BackendUserMorphType) with one public attachOne avatar
  (cabana.BackendUserAvatarField), as WinterCMS stores it
- POST /account/avatar stores one file_data image (jpg, jpeg, png, gif or
  webp, image guard, at most cabana.AvatarMaxBytes = 2 MiB, under a
  server-generated name) and replaces the previous avatar: old rows are
  deleted in the transaction, their blobs after the commit
- DELETE /account/avatar removes the avatar and is idempotent
- GET /auth/me and the account routes return avatar {url, thumb_url}
  (250x250 crop); cabana.BackendUserAvatar lets plugins read it
- swag annotations, regenerated admin.json and schema.d.ts; route
  inventories, CSRF walk (29) and OpenAPI conformance learn both routes
- SPA: avatar upload and remove on My account, avatar image in the user
  menu trigger and header; backend::lang.myaccount avatar strings
- rebuilt boardwalk dist; cabana README, users-and-permissions and
  admin-spa docs

docs/backend/admin-spa.md

@@ -33,7 +33,7 @@ A list row that carries a state (deleted, negative or disabled) shows a text bad
 
 A form whose schema carries `preview` has a read-only record screen at `<controller>/<id>/preview`, next to the list (`<controller>`), the create form (`<controller>/create`) and the update form (`<controller>/<id>`). It renders the fields whose context allows `preview` as text, the status hint partial above them and, in its footer, the record actions the record response offers before the one edit button. Opening that route for a form without a preview goes to the update form. See [Preview screen](forms.md#preview-screen).
 
-The user menu in the top bar has a My account item that opens `<prefix>/account`. That screen edits the signed-in administrator's own first name, last name, email and subtitle, shows the login read-only, and changes the password when a new one is typed together with the current one. Server validation errors show on their fields, and a successful save updates the name in the top bar without a reload. See [My account](users-and-permissions.md#my-account).
+The user menu in the top bar has a My account item that opens `<prefix>/account`. That screen edits the signed-in administrator's own first name, last name, email and subtitle, uploads and removes their avatar, shows the login read-only, and changes the password when a new one is typed together with the current one. Server validation errors show on their fields, and a successful save updates the name and the avatar in the top bar without a reload. See [My account](users-and-permissions.md#my-account).
 
 ## Types from OpenAPI