Show client avatars and let the user set their own

Avatars are fetched from the server's file repository as /avatar_<id>,
verified against client_flag_avatar and cached on disk; animated GIFs play in
the info panel. Self > Set avatar... uploads or removes our own picture within
the server's size limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ts3-client/README.md

@@ -71,6 +71,11 @@ frontend supplies its own UI and audio backend while reusing `core` unchanged.
   **private chat**, or **locally mute** them.
 - **Chat** to the current channel or the whole server; receive channel/server/private
   messages and **pokes**.
+- **Avatars** — a selected client's avatar is shown in the info panel, fetched from the
+  server's file repository (`/avatar_<id>` in channel 0, verified against the client's
+  `client_flag_avatar` MD5) and cached on disk; **animated GIFs play**. Self → Set
+  avatar… uploads your own picture (PNG/JPEG/GIF/BMP, within the server's
+  `i_client_max_avatar_filesize`) or removes it.
 - **Server bookmarks** — quick-connect menu with add/edit/remove management, each
   optionally pinned to a specific identity.
 - **Identity management** (Tools → Identities) — keep several identities, mark one