Retire the Arch package, document AppImage desktop integration

The `triple-c-bin` package was never on the AUR, so installing it meant
downloading a file and running `pacman -U` — the same gesture as making an
AppImage executable, for a second artifact to keep building. And being
`workflow_dispatch`-only it reached 1 release in 28 (only v0.4.16 has a
`.pkg.tar.zst`), while HOW-TO-USE.md told Arch and CachyOS users to download
it from every release. A distribution channel that is absent 27 times out of
28 is worse than not promising one.

`packaging/arch/` and `.gitea/workflows/publish-arch-package.yml` are
preserved whole on `hold/arch-packaging`, the same way the disk panel and
drag-out work were held rather than deleted. What would make an Arch package
worth having is an AUR account and its SSH key as a repo secret — both
one-time manual steps that never happened; the workflow's own header already
said as much about its AUR push step.

This also closes the gap that prompted the review: nothing validated the
PKGBUILD until someone manually dispatched the workflow, making it the only
packaging path with no CI coverage. Removing it removes the untested surface
rather than adding a job to test something nobody installs.

In its place, `scripts/install-appimage.sh` does what a package manager's
install hooks would. An AppImage carries a `.desktop` entry and icons inside
itself, but nothing on the host reads them, so it never appears in the app
launcher. The script extracts the bundled icons into the user's icon theme
and writes a launcher entry — no sudo, nothing outside `~/.local/share`, and
the AppImage itself is never copied or moved.

Two details it gets right on purpose:

  * The `Exec` line is rewritten, not copied. The bundled entry says
    `Exec=triple-c`, which resolves only inside the running AppImage's own
    mount — a verbatim copy gives a launcher entry that starts nothing.
  * Extraction uses `--appimage-extract`, which needs no FUSE, so the script
    works on a machine where *running* the AppImage would first need
    `fuse2` installed. That requirement is now documented too: Arch and
    CachyOS do not ship FUSE 2 by default.

Verified against the real artifact — the AppImage from this repo's own
preview-3a49a67 release: 4 icon sizes install, `desktop-file-validate` passes
with no warnings, `--uninstall` leaves nothing behind, and shellcheck is
clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ApLYH6ybHwQFkMCtKuHrrV

CLAUDE.md

@@ -677,6 +677,26 @@ deliberately out of scope — this is not a project backup.
   specifically to make them unmissable — the frontend's `<li>`/warning boxes also get `break-all`
   as a second layer against the same failure mode.
 
+## Packaging
+
+Linux ships as `.deb`, `.rpm` and AppImage, all three built by `build-app.yml` (releases) and
+`build-app-preview.yml` (the PR check). **There is deliberately no Arch package.** A
+`triple-c-bin` `PKGBUILD` and a `publish-arch-package.yml` existed and were removed; they live on
+`hold/arch-packaging`. Do not re-add them without the piece that was always missing: the package
+was never on the AUR, so it was a manual `pacman -U` of a downloaded file — the same gesture as
+the AppImage, for a second artifact to keep working. Being `workflow_dispatch`-only it also
+reached 1 release in 28, while `HOW-TO-USE.md` told Arch users to download it from every release.
+An AUR account and its SSH key as a repo secret are what would make it worth having; until then
+the AppImage is the Arch story.
+
+`scripts/install-appimage.sh` is the desktop-integration half, and it exists because an AppImage
+has no installer: it extracts the bundled icons into `~/.local/share/icons/hicolor` and writes a
+`.desktop` entry. It **rewrites** the `Exec` line rather than copying the bundled entry — the
+bundled one is `Exec=triple-c`, which resolves only inside the AppImage's own mount, so a
+verbatim copy yields a launcher entry that starts nothing. It keeps `StartupWMClass` exactly as
+the bundle sets it, which is what lets the shell match the window to the entry. Extraction uses
+`--appimage-extract`, which needs no FUSE, so the script works before `fuse2` is installed.
+
 ## Testing
 
 Frontend tests use Vitest with jsdom environment and React Testing Library. Setup file at `src/test/setup.ts`. Run a single test file: