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

docs/admin-guide.md

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+# Admin guide
+
+## Deploying the panel
+
+```bash
+cp .env.example .env         # set ADMIN_PASSWORD
+docker compose up -d
+docker compose logs -f panel # first start prints the admin account
+```
+
+Everything the panel stores lives in the `panel-data` volume (`/data`):
+
+| Path | What |
+|---|---|
+| `panel.db` | SQLite database (users, instances, settings, stats) |
+| `files/<instance>/` | Configs and uploads hosted for launchers |
+| `uploads/` | Logos, backgrounds, icons |
+| `jwt.secret` | Token signing key (unless `JWT_SECRET` is set) |
+
+**Backups:** stop the container (or use `sqlite3 panel.db ".backup backup.db"`) and copy the volume.
+
+**Updating:** `docker compose pull && docker compose up -d`. Database migrations run automatically.
+
+### HTTPS
+
+Launchers talk to the panel over the internet, so serve it over HTTPS. Example with Caddy:
+
+```caddyfile
+panel.example.com {
+    reverse_proxy localhost:8080
+}
+```
+
+Large modpack uploads go through the proxy — raise its body-size limit if needed (nginx: `client_max_body_size 2g;`).
+
+## Instances
+
+An instance = one playable profile: a Minecraft version, an optional mod loader, files (mods, configs…) and an optional server.
+
+- **Version** — choose any Minecraft version and Vanilla / Fabric / Quilt / Forge / NeoForge. Leave the loader version empty for the latest recommended build (it's pinned when you save).
+- **Modrinth** — search, pick a version, *Use*. Mods are downloaded by players straight from Modrinth's CDN; configs (`overrides/`) are hosted by the panel.
+- **CurseForge** — needs a free API key from [console.curseforge.com](https://console.curseforge.com) (Settings → Integrations, or `CURSEFORGE_API_KEY`). Some authors block third-party downloads; those files show up under **Files** with a link — download them yourself and upload them into the same folder.
+- **Upload .zip** — a `.mrpack`, a CurseForge export, or any zipped instance folder (with `mods/`, `config/`, … or a `.minecraft/` inside). For plain zips, pick the Minecraft version and loader.
+- **Files** — add or remove individual files at any time. Files players add themselves are never touched; files you remove are deleted from players' instances on their next launch.
+
+Every save bumps the instance **revision**; launchers re-verify files when it changes, otherwise launching is instant.
+
+### Built-in server
+
+On the **Server** tab: name, address, port.
+
+- *Add to multiplayer list* writes the server into `servers.dat` (keeping servers players added).
+- *Join automatically* connects on start (Quick Play on 1.20+, `--server` on older versions).
+
+Your server must allow the accounts you use: offline-mode (`online-mode=false`) for panel/offline accounts, or online-mode for Microsoft accounts. Protect offline-mode servers with a whitelist or an auth plugin.
+
+### Access
+
+- **Everyone** — any launcher, including offline and Microsoft accounts.
+- **Signed-in players** — any panel account.
+- **Specific groups** — members of the selected groups (create groups on the Players page). Admins see everything.
+
+## Players
+
+- **Sign-ups:** Settings → *Closed* (admins create accounts), *Needs approval*, or *Open*.
+- Panel accounts play under their username with the offline-mode UUID an offline server computes, so inventories and permissions stay consistent.
+- Disabling an account signs it out everywhere on the next request.
+- Ten failed logins lock an account for five minutes.
+
+## Launcher design
+
+Everything on this page is pushed to launchers when they start or refresh — no reinstall. The **Features** tab lets you allow or block players from changing the theme or Java settings. **Custom CSS** is injected last; handy variables are `--accent`, `--accent-2`, `--surface`, `--radius`.
+
+## Releasing the launcher
+
+See the README's *Build your branded launcher*. Players get updates automatically: the launcher checks GitHub Releases on start and offers the new installer.
+
+**Code signing:** unsigned installers trigger a Windows SmartScreen warning ("More info → Run anyway"). To avoid it, sign the installer with a code-signing certificate (e.g. Azure Trusted Signing) — Tauri supports this via `bundle.windows.signCommand`.