Add a native Arch/CachyOS package via its own AUR publish workflow

Part of triple-c#34's third ask ("I would like to also have an
Arch/CachyOS native version as well"), addressed separately from the
Wayland crash fix (fix/wayland-webkit-egl-crash) since it's an unrelated
feature, not a bug.

packaging/arch/PKGBUILD is a "-bin" AUR package repackaging the same .deb
build-app.yml already produces — no Rust/Node toolchain needed to install
it, and the user gets exactly the binary the project ships and tests.
Verified end to end against a real release (v0.4.14) rather than going by
Tauri's generic docs: downloaded the actual .deb, ldd'd the actual binary
to ground-truth `depends` (dropped `pango` and `libayatana-appindicator`
from an earlier draft — the first is already pulled in transitively by
gtk3, the second was never linked at all since this app has no tray icon
or menu), and ran a real makepkg/namcap/pacman -U cycle. namcap caught a
real issue this way (missing license file under
/usr/share/licenses/triple-c-bin/), now fixed by fetching LICENSE
alongside the .deb.

.gitea/workflows/publish-aur-package.yml does the actual publishing:
given a version (or "latest"), it finds that release's real Linux asset
on GitHub, downloads it, computes real checksums, renders the PKGBUILD
template, validates the result with makepkg and namcap inside a real
Arch container, and pushes to AUR. workflow_dispatch only, deliberately —
the same reasoning that killed sync-release.yml in triple-c#32 (releases
are assembled by build-app.yml across three separate platform jobs, so
there's no single automatic event that fires only once the Linux .deb
this needs actually exists) applies here too.

Requires a repo secret this workflow cannot set up itself:
AUR_SSH_PRIVATE_KEY, from an AUR account that has already created (or
been given co-maintainer access to) triple-c-bin — both one-time manual
steps on aur.archlinux.org. Until that secret exists, the workflow fails
loudly at the push step rather than silently doing nothing. See
packaging/arch/README.md for the full maintenance flow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FGjXq6fqtAFHdbhk4f3PfZ

TECHNICAL.md

@@ -412,12 +412,18 @@ triple-c/
 │
 ├── .gitea/
 │   └── workflows/
-│       ├── build-app.yml          # Build Tauri app (Linux/macOS/Windows); mirrors releases to GitHub inline
-│       ├── build-app-preview.yml  # Preview builds
-│       ├── build.yml              # Build container image (multi-arch)
-│       ├── build-stt.yml          # Build the STT image
-│       ├── backfill-releases.yml  # Bulk copy releases to GitHub
-│       └── cleanup-releases.yml   # Prune old releases
+│       ├── build-app.yml            # Build Tauri app (Linux/macOS/Windows); mirrors releases to GitHub inline
+│       ├── build-app-preview.yml    # Preview builds
+│       ├── build.yml                # Build container image (multi-arch)
+│       ├── build-stt.yml            # Build the STT image
+│       ├── backfill-releases.yml    # Bulk copy releases to GitHub
+│       ├── cleanup-releases.yml     # Prune old releases
+│       └── publish-aur-package.yml  # Publish triple-c-bin to the AUR (packaging/arch/)
+│
+├── packaging/
+│   └── arch/                      # AUR triple-c-bin package — see packaging/arch/README.md
+│       ├── PKGBUILD
+│       └── README.md
 │
 └── app/                           # Tauri v2 desktop application
     ├── package.json               # React, xterm.js, zustand, tailwindcss