Documentation site: mdBook user + contributor guide (2.2)

An mdBook under docs/ rendering eight pages: introduction,
installation, first-run/config, downloading, an anti-bot guide that
captures the hard-won cookies/curl_cffi/POT/player-client knowledge,
troubleshooting (the nine error classes + non-download issues),
architecture (the two-front-ends/one-engine design, settings flow,
layout invariant, persistence), and packaging.

- .forgejo/workflows/docs.yml builds the book and publishes docs/book/
  to the `pages` branch for Codeberg Pages on docs/ changes.
- docs/book/ is gitignored (rendered output).
- README: fix the stale backup.directory comment — every platform nests
  under the one library root, not a channels/ sibling split.
- Add CLAUDE.md (repo guidance for Claude Code).

Builds clean with mdbook 0.5; intra-doc anchor links verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/src/packaging.md

@@ -0,0 +1,39 @@
+# Packaging
+
+Build distributable Linux packages with one script:
+
+```sh
+scripts/package.sh all       # .deb + .rpm + .AppImage → dist/
+scripts/package.sh deb       # just the .deb
+scripts/package.sh rpm
+scripts/package.sh appimage
+```
+
+It builds the release binary once and reuses it for every format,
+installing `cargo-deb` / `cargo-generate-rpm` on demand and downloading
+`appimagetool` to `dist/tools/` on first AppImage build. Per-format
+failures are isolated and summarized at the end. Output (gitignored)
+lands in `dist/`.
+
+## Formats
+
+- **`.deb`** — built by [cargo-deb](https://github.com/kornelski/cargo-deb)
+  from `[package.metadata.deb]` in `Cargo.toml`.
+- **`.rpm`** — built by
+  [cargo-generate-rpm](https://github.com/cat-in-136/cargo-generate-rpm)
+  from `[package.metadata.generate-rpm]`. (`ffmpeg` on Fedora is in
+  RPM Fusion.)
+- **AppImage** — a hand-rolled AppDir + appimagetool. Bundles the GUI
+  binary's shared-library closure only; `yt-dlp`/`ffmpeg`/`mpv` stay host
+  PATH deps, same as the package declarations.
+- **Arch** — use the repo's `PKGBUILD` (not this script); run `makepkg`
+  from a clean directory.
+
+## CI
+
+`.forgejo/workflows/release.yml` runs `scripts/package.sh all` on every
+pushed `v*` tag and attaches the artifacts to the Codeberg release.
+`.forgejo/workflows/test.yml` runs the full test suite on every push.
+
+The repo's [`docs/PACKAGING.md`](https://codeberg.org/anassaeneroi/yt-offline/src/branch/main/docs/PACKAGING.md)
+has the per-distro install commands and the Windows/macOS status in full.