Per-distro packaging: .deb / .rpm / AppImage + CI (1.8)

Ships beyond the Arch PKGBUILD so non-Arch users can install without a
Rust toolchain.

## Package metadata (Cargo.toml)

- [package.metadata.deb] for cargo-deb: declares the runtime subprocess
  deps (yt-dlp, ffmpeg, mpv, xdg-utils) that auto-detection can't find,
  plus libxcb1/libc6 link deps and libnotify4 as recommended.
- [package.metadata.generate-rpm] for cargo-generate-rpm with the
  RPM-distro dep names.
- Filled in license / repository / authors / readme package fields.

## scripts/package.sh

One entry point: `scripts/package.sh [deb|rpm|appimage|all]`. Builds the
release binary once and reuses it. Installs cargo-deb / cargo-generate-rpm
on demand; downloads appimagetool to dist/tools/ on first AppImage build.
Per-format failures are isolated and summarized at the end rather than
aborting the run. Output to dist/ (gitignored).

AppImage is hand-rolled (AppDir + AppRun + root desktop/icon) and bundles
only the GUI binary's shared-lib closure — yt-dlp/ffmpeg/mpv stay host
PATH deps, same as the package declarations.

## CI

.forgejo/workflows/release.yml builds all formats on a v* tag push and
attaches them to the Codeberg release via actions/forgejo-release. Manual
workflow_dispatch builds without uploading for smoke testing. Sets
APPIMAGE_EXTRACT_AND_RUN since CI containers lack FUSE.

## docs/PACKAGING.md

Per-format build + install instructions, the Arch PKGBUILD pointer, and
an honest Windows/macOS section: both are blocked on the Linux-only tray
(ksni) + file-picker (rfd xdg-portal) deps needing per-OS abstraction.
The rest of the stack already cross-compiles.

Verified .deb (dpkg control + payload correct) and .rpm (payload correct)
build cleanly through the script on this host.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -4,6 +4,9 @@
 # Rust build artifacts
 /target/
 
+# Packaging output (.deb / .rpm / .AppImage + downloaded appimagetool)
+/dist/
+
 # makepkg build artifacts (finished .pkg.tar.zst is NOT ignored)
 /pkg/
 /src/yt-offline/