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

modules/cabana/admin_openapi.go

@@ -108,6 +108,15 @@ type AdminProfile struct {
 	Subtitle    string            `json:"subtitle"`
 	IsSuperuser bool              `json:"is_superuser"`
 	Role        *AdminRoleSummary `json:"role,omitempty"`
+	// Avatar is present when the admin has a public avatar.
+	Avatar *AdminAvatar `json:"avatar,omitempty"`
+}
+
+// AdminAvatar is an admin's avatar: the public URL of the image and of its
+// 250x250 crop thumbnail (left out when the thumbnail cannot be made).
+type AdminAvatar struct {
+	URL      string `json:"url"`
+	ThumbURL string `json:"thumb_url,omitempty"`
 }
 
 // AdminAccountRequest is the body of PUT /account. It documents the only keys
@@ -181,7 +190,7 @@ func AdminLogout() {}
 // AdminMe documents GET /auth/me.
 //
 // @Summary Current admin
-// @Description The signed-in admin's profile: login, email, names, the subtitle (backend_users.metadata.subtitle, "" when not set), the superuser flag and the role.
+// @Description The signed-in admin's profile: login, email, names, the subtitle (backend_users.metadata.subtitle, "" when not set), the superuser flag, the role and, when the admin has one, the avatar with its public url and the 250x250 crop thumb_url.
 // @Tags admin
 // @Produce json
 // @Security BackendBearer
@@ -209,6 +218,36 @@ func AdminMe() {}
 // @Router /account [put]
 func AdminAccountUpdate() {}
 
+// AdminAccountAvatarUpload documents POST /account/avatar.
+//
+// @Summary Upload my avatar
+// @Description Stores one multipart file_data part as the signed-in admin's avatar, a public attachOne (system_files attachment_type Backend\Models\User, field avatar) under a server-generated name, and replaces the previous avatar. Only jpg, jpeg, png, gif and webp images that pass the image check, of at most 2 MiB, are accepted; anything else is a 422 on avatar. A body over the upload cap answers 413 payload_too_large.
+// @Tags admin
+// @Accept multipart/form-data
+// @Produce json
+// @Security BackendBearer
+// @Param file_data formData file true "Image (jpg, jpeg, png, gif or webp, at most 2 MiB)"
+// @Success 200 {object} Envelope[AdminProfile]
+// @Failure 401 {object} ErrorEnvelope
+// @Failure 403 {object} ErrorEnvelope
+// @Failure 413 {object} ErrorEnvelope
+// @Failure 422 {object} ErrorEnvelope
+// @Router /account/avatar [post]
+func AdminAccountAvatarUpload() {}
+
+// AdminAccountAvatarRemove documents DELETE /account/avatar.
+//
+// @Summary Remove my avatar
+// @Description Removes the signed-in admin's avatar and its files. Removing an avatar that is not there also answers 200.
+// @Tags admin
+// @Produce json
+// @Security BackendBearer
+// @Success 200 {object} Envelope[AdminProfile]
+// @Failure 401 {object} ErrorEnvelope
+// @Failure 403 {object} ErrorEnvelope
+// @Router /account/avatar [delete]
+func AdminAccountAvatarRemove() {}
+
 // AdminNavigation documents GET /navigation.
 //
 // @Summary Admin navigation