Launcher: panel accounts via authlib-injector, skin & cape editor

Microsoft sign-in is gone. Server accounts now receive Yggdrasil tokens
from the panel and launch with authlib-injector pointed at the panel's
auth server, so online-mode servers verify players against it.

- Download authlib-injector from the panel mirror (fallback: official),
  SHA-256 verified and cached, with prefetched metadata
- New Skin & cape settings tab: upload, arm style, reset, cape picker
  with front/back previews
- Avatars render from the panel's head endpoint

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ARcGWxLx21FwXJ3yfGriS

launcher/src-tauri/Cargo.toml

@@ -18,7 +18,7 @@ anyhow.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 tokio.workspace = true
-reqwest.workspace = true
+reqwest = { workspace = true, features = ["multipart"] }
 futures.workspace = true
 uuid.workspace = true
 base64.workspace = true