Add launcher engine (scopenet-core) and admin panel server

- scopenet-shared: wire types for branding, auth, instances, manifests
- scopenet-core: Mojang versions/libraries/assets, automatic Java runtimes,
  Fabric/Quilt/Forge/NeoForge installation, file sync, launch command
  builder, Microsoft auth, servers.dat injection, server list ping
- scopenet-panel: Axum + SQLite admin API with JWT auth, users/groups,
  branding, settings, instances, Modrinth/CurseForge/zip import, file
  hosting and launch stats

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

panel/server/src/error.rs

@@ -0,0 +1,74 @@
+use axum::http::StatusCode;
+use axum::response::{IntoResponse, Response};
+use axum::Json;
+use serde_json::json;
+
+/// Every handler returns `Result<_, AppError>`; errors become
+/// `{"error": "..."}` with a sensible status code.
+#[derive(Debug)]
+pub struct AppError {
+    pub status: StatusCode,
+    pub message: String,
+}
+
+pub type AppResult<T> = Result<T, AppError>;
+
+impl AppError {
+    pub fn new(status: StatusCode, message: impl Into<String>) -> Self {
+        Self { status, message: message.into() }
+    }
+    pub fn bad_request(m: impl Into<String>) -> Self {
+        Self::new(StatusCode::BAD_REQUEST, m)
+    }
+    pub fn unauthorized(m: impl Into<String>) -> Self {
+        Self::new(StatusCode::UNAUTHORIZED, m)
+    }
+    pub fn forbidden(m: impl Into<String>) -> Self {
+        Self::new(StatusCode::FORBIDDEN, m)
+    }
+    pub fn not_found(m: impl Into<String>) -> Self {
+        Self::new(StatusCode::NOT_FOUND, m)
+    }
+    pub fn conflict(m: impl Into<String>) -> Self {
+        Self::new(StatusCode::CONFLICT, m)
+    }
+}
+
+impl IntoResponse for AppError {
+    fn into_response(self) -> Response {
+        if self.status.is_server_error() {
+            tracing::error!("{}", self.message);
+        }
+        (self.status, Json(json!({ "error": self.message }))).into_response()
+    }
+}
+
+impl From<anyhow::Error> for AppError {
+    fn from(e: anyhow::Error) -> Self {
+        Self::new(StatusCode::BAD_GATEWAY, format!("{e:#}"))
+    }
+}
+
+impl From<sqlx::Error> for AppError {
+    fn from(e: sqlx::Error) -> Self {
+        Self::new(StatusCode::INTERNAL_SERVER_ERROR, format!("database error: {e}"))
+    }
+}
+
+impl From<std::io::Error> for AppError {
+    fn from(e: std::io::Error) -> Self {
+        Self::new(StatusCode::INTERNAL_SERVER_ERROR, format!("io error: {e}"))
+    }
+}
+
+impl From<serde_json::Error> for AppError {
+    fn from(e: serde_json::Error) -> Self {
+        Self::bad_request(format!("invalid JSON: {e}"))
+    }
+}
+
+impl From<axum::extract::multipart::MultipartError> for AppError {
+    fn from(e: axum::extract::multipart::MultipartError) -> Self {
+        Self::bad_request(format!("upload failed: {e}"))
+    }
+}