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

Cargo.toml

@@ -38,3 +38,6 @@ strip = true
 
 [profile.dev.package."*"]
 opt-level = 1
+# Dependencies rarely need a debugger; skipping their debug info keeps
+# target/ several times smaller.
+debug = false