Panel: Yggdrasil auth server (authlib-injector), skins and capes

- Yggdrasil API per the authlib-injector spec: metadata with signing key
  and skin domains, authenticate/refresh/validate/invalidate/signout,
  join/hasJoined, profile lookup, texture upload, and the minecraftservices
  endpoints (chat certificates, publickeys, attributes, blocklist)
- 4096-bit signing key generated once into the data volume; textures and
  chat certificates signed SHA1withRSA (verified with Java's own crypto)
- Skins/capes stored content-addressed after validation and re-encoding;
  cape library with public/group/private visibility; head avatars API
- Launcher login returns a game session; launcher sessions recorded for
  launcher-only servers; authlib-injector download mirror
- Schema v2: player UUIDs (offline UUID backfilled), skins, capes, tokens,
  sessions, chat keys, game server tables
- Remove Microsoft sign-in from the engine and panel

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

panel/server/src/store.rs

@@ -27,6 +27,9 @@ pub struct Settings {
     pub curseforge_api_key: Option<String>,
     /// Where players can download the launcher (shown on the dashboard).
     pub launcher_download_url: Option<String>,
+    /// Public address of the panel (e.g. https://panel.example.com). Used in
+    /// skin URLs and the auth server metadata. Falls back to the request.
+    pub public_url: Option<String>,
 }
 
 pub async fn settings(state: &AppState) -> AppResult<Settings> {