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/src/game.rs

@@ -131,7 +131,20 @@ pub async fn run(app: AppHandle, instance_id: String, start: bool, deep: bool) -
         emit_state(&app, &instance_id, "idle", Some("Instance is up to date".into()));
         return Ok(());
     }
-    let auth = auth.unwrap();
+    let (auth, auth_server) = auth.unwrap();
+
+    // Panel accounts authenticate through the panel's Yggdrasil server via
+    // authlib-injector; offline accounts launch without it.
+    let agent_args = match &auth_server {
+        Some(api) => {
+            report(Event::Stage { stage: Stage::Launching, label: "Preparing sign-in".into() });
+            let jar =
+                scopenet_core::authlib::ensure(&state.http, &state.layout, Some(&panel)).await.context("setting up authlib-injector")?;
+            let prefetched = scopenet_core::authlib::prefetch_metadata(&state.http, api).await.ok();
+            scopenet_core::authlib::jvm_args(&jar, api, prefetched.as_deref())
+        }
+        None => vec![],
+    };
 
     report(Event::Stage { stage: Stage::Launching, label: "Starting Minecraft".into() });
     let pick = |a: u32, b: u32, c: u32| {
@@ -153,6 +166,7 @@ pub async fn run(app: AppHandle, instance_id: String, start: bool, deep: bool) -
     let branding = state.manifest.read().unwrap().as_ref().map(|m| m.branding.clone()).unwrap_or_default();
     let opts = LaunchOptions {
         auth: auth.clone(),
+        agent_args,
         game_dir: game_dir.clone(),
         memory_min_mb: min,
         memory_max_mb: max,