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

crates/core/src/sys.rs

@@ -0,0 +1,41 @@
+//! Tiny system probes (kept dependency-free to stay lightweight).
+
+/// Total physical memory in MiB (best effort; 8 GiB if unknown).
+pub fn total_memory_mb() -> u64 {
+    imp().unwrap_or(8192)
+}
+
+#[cfg(windows)]
+fn imp() -> Option<u64> {
+    use windows_sys::Win32::System::SystemInformation::{GlobalMemoryStatusEx, MEMORYSTATUSEX};
+    let mut status: MEMORYSTATUSEX = unsafe { std::mem::zeroed() };
+    status.dwLength = std::mem::size_of::<MEMORYSTATUSEX>() as u32;
+    let ok = unsafe { GlobalMemoryStatusEx(&mut status) };
+    (ok != 0).then(|| status.ullTotalPhys / 1024 / 1024)
+}
+
+#[cfg(target_os = "linux")]
+fn imp() -> Option<u64> {
+    let info = std::fs::read_to_string("/proc/meminfo").ok()?;
+    let line = info.lines().find(|l| l.starts_with("MemTotal:"))?;
+    let kb: u64 = line.split_whitespace().nth(1)?.parse().ok()?;
+    Some(kb / 1024)
+}
+
+#[cfg(not(any(windows, target_os = "linux")))]
+fn imp() -> Option<u64> {
+    None
+}
+
+/// Recursively sum file sizes under `path`.
+pub fn dir_size(path: &std::path::Path) -> u64 {
+    let Ok(entries) = std::fs::read_dir(path) else { return 0 };
+    entries
+        .filter_map(|e| e.ok())
+        .map(|e| match e.file_type() {
+            Ok(t) if t.is_dir() => dir_size(&e.path()),
+            Ok(t) if t.is_file() => e.metadata().map(|m| m.len()).unwrap_or(0),
+            _ => 0,
+        })
+        .sum()
+}