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/installation.md

@@ -0,0 +1,73 @@
+# Installation
+
+yt-offline is a single Rust binary. You can install a prebuilt package,
+build from source, or grab the AppImage.
+
+## Runtime dependencies
+
+Whichever way you install, these are invoked as subprocesses at runtime:
+
+- **yt-dlp** — the download engine. You can use the system one *or* let
+  yt-offline manage a bundled copy (see [First run](./first-run.md)).
+- **ffmpeg** — muxing, format conversion, on-the-fly transcode for the
+  web player.
+- **mpv** — the default desktop player (any player taking a file path
+  works; set it in Settings).
+- **xdg-utils** — `xdg-open` for "Show in file manager".
+
+## Prebuilt packages (Linux)
+
+Releases attach `.deb`, `.rpm`, and `.AppImage` artifacts.
+
+```sh
+# Debian / Ubuntu / Mint
+sudo apt install ./yt-offline_*_amd64.deb
+
+# Fedora / RHEL / openSUSE   (ffmpeg via RPM Fusion)
+sudo dnf install ./yt-offline-*.x86_64.rpm
+
+# Any Linux — AppImage
+chmod +x yt-offline-*-x86_64.AppImage
+./yt-offline-*-x86_64.AppImage
+```
+
+## Arch / CachyOS / Manjaro
+
+A `PKGBUILD` ships in the repo root. Build it from a **clean** directory:
+
+```sh
+mkdir build && cd build
+cp /path/to/repo/PKGBUILD .
+makepkg -si
+```
+
+For repeated builds after pulling new commits, always pass `-C`
+(cleanbuild) so makepkg re-checks out the latest source instead of
+reusing a stale cached clone.
+
+## From source
+
+```sh
+# Debian/Ubuntu build deps
+sudo apt install build-essential pkg-config curl git python3-venv \
+  libxcb-render0-dev libxcb-shape0-dev libxcb-xfixes0-dev \
+  libxkbcommon-dev libssl-dev
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+source "$HOME/.cargo/env"
+
+git clone https://codeberg.org/anassaeneroi/yt-offline
+cd yt-offline
+cargo build --release
+./target/release/yt-offline           # desktop GUI
+./target/release/yt-offline --web 8080  # headless web server
+```
+
+`python3-venv` is only needed for the bundled-yt-dlp install path; skip
+it if you'll always use system yt-dlp.
+
+## Windows / macOS
+
+Not first-class yet. The Linux-only system tray (`ksni`) and file dialog
+(`rfd` xdg-portal) need per-OS backends before a clean cross-build; the
+rest of the stack already compiles. See the
+[architecture notes](./architecture.md#platform-support).