Add Docker image, Compose, CI/CD workflows and docs

- Multi-arch (amd64/arm64) panel image: static musl binary on scratch,
  cross-compiled with cargo-zigbuild (no QEMU), non-root, healthcheck
- docker-compose.yml + .env.example for one-command deployment
- CI: fmt, clippy, tests, web type-checks, Windows launcher build,
  real-network installs of vanilla/Fabric/Quilt/Forge/NeoForge,
  Docker build
- Release: Windows NSIS installer with baked-in panel URL, GHCR image
  push and GitHub release (tag push or manual dispatch)
- README, admin guide, Microsoft auth setup, architecture, development
- rustfmt config and formatting pass

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

crates/core/src/http.rs

@@ -129,10 +129,8 @@ async fn try_download(client: &reqwest::Client, dl: &Download, on_bytes: &(dyn F
         tokio::fs::create_dir_all(dir).await?;
     }
     let resp = client.get(&dl.url).send().await?.error_for_status()?;
-    let part = dl.dest.with_extension(format!(
-        "{}part",
-        dl.dest.extension().map(|e| format!("{}.", e.to_string_lossy())).unwrap_or_default()
-    ));
+    let part =
+        dl.dest.with_extension(format!("{}part", dl.dest.extension().map(|e| format!("{}.", e.to_string_lossy())).unwrap_or_default()));
     let mut file = tokio::fs::File::create(&part).await?;
     let mut hasher = Sha1::new();
     let mut stream = resp.bytes_stream();