Add reproducible build system, native Unraid plugin, and WebUI

- versions.env pins podman, conmon, crun, netavark, aardvark-dns, passt,
  and fuse-overlayfs to verified upstream source checksums; SlackBuild
  recipes, scripts/build-packages.sh, checksums.sh, release.sh, and
  update-versions.sh implement the reproducible pipeline; GitHub Actions
  workflows build in a Slackware container and publish releases without
  committing any binaries.

- plugin/podman.plg installs/updates/removes all eight packages (the
  seven components plus the plugin's own unraid-podman scaffolding
  package) via upgradepkg, using the official Unraid array-event hook
  mechanism (event/disks_mounted, event/stopping) instead of editing
  /boot/config/go. rc.podman and the sbin/ helper scripts implement
  storage creation, config seeding/sync, preflight checks, autostart
  with per-container Safe-Mode, and package verify/update/rollback.

- webui/plugins/podman implements the Dashboard, Containers, Pods,
  Images, Volumes, Networks, Logs, Terminal, Compose, and Settings
  panels against the approved mockup (webui/mockups/prototype.html),
  talking to podman system service exclusively via PodmanClient.php
  (libpod REST API over the Unix socket), with two documented
  exceptions: Terminal's one-shot exec model and Compose's use of the
  podman compose CLI, since libpod has no REST equivalent for either.

- docs/ARCHITECTURE.md and docs/ROADMAP.md record the design decisions
  and honest current status (syntax-checked, unit- and
  integration-tested against fake sockets/servers; not yet run against
  a real Unraid/Podman/Slackware system).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/INSTALL.md

@@ -0,0 +1,39 @@
+# Installation Guide (draft)
+
+> This guide is a placeholder. Nothing described here is functional yet — the
+> project is at the architecture/scaffolding stage. See
+> [ARCHITECTURE.md](ARCHITECTURE.md) and [ROADMAP.md](ROADMAP.md).
+
+## Planned installation flow
+
+Once a release exists, installation will follow standard Unraid conventions:
+
+1. In the Unraid WebUI, go to **Plugins → Install Plugin**.
+2. Paste the `.plg` URL (or install via Community Applications, once listed).
+3. Unraid downloads and runs `podman.plg`, which:
+   - installs the Slackware `.txz` packages under `packages/`,
+   - stages default configuration under `/boot/config/plugins/podman/`,
+   - registers the boot hook in `/boot/config/go`,
+   - starts the Podman service for the first time.
+
+## Planned requirements
+
+- Unraid version: TBD minimum (depends on kernel/cgroup v2 requirements — see
+  [ARCHITECTURE.md, section 5.3](ARCHITECTURE.md#53-abhängigkeitsprüfung)).
+- A cache pool (recommended) or a dedicated array disk path for
+  `podman.img` — see [ARCHITECTURE.md, section 4.3](ARCHITECTURE.md#43-persistente-nutzdaten-cache-pool-bevorzugt-array-als-fallback).
+- Sufficient free space on that pool/disk for the container storage image.
+
+## Uninstallation (planned)
+
+Removing the plugin removes the installed packages and init hooks but
+**preserves** data under `/mnt/*/system/podman/` and
+`/boot/config/plugins/podman/` by default. A separate, explicit option will be
+provided for full data removal.
+
+## Verifying the installation (planned)
+
+```sh
+rc.podman status
+podman info
+```