desktop: drop the redundant deb depends, correct the pacman docs

Follow-up to 8a8b2b1, driven by what run 2872's new checks actually printed.

The .deb verification did its job on its first run: tauri already infers
exactly libwebkit2gtk-4.1-0 + libgtk-3-0, so declaring the same two in
tauri.conf.json produced a control file listing each of them twice. Removed
the declaration — verify.sh is the real guard, and it fails the build if
inference ever stops covering what the binary links.

The pacman step revealed ci-tauri carries neither zstd nor bsdtar, so packages
currently ship as .pkg.tar.xz with no .MTREE. Both are working outcomes
(pacman reads xz; only `pacman -Qkk` needs .MTREE), but the docs promised
.zst, so the README, the release notes and ci-requirements.md now describe
what the build actually produces.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SreJkbxB4gx8pPsu8QbLPi

desktop/packaging/arch/README.md

@@ -1,7 +1,7 @@
 # ThoughtSync desktop — Arch package
 
-A **prebuilt** native pacman package (`.pkg.tar.zst`), published as an asset on
-every ThoughtSync release. Nothing to compile, no toolchain to install.
+A **prebuilt** native pacman package, published as an asset on every ThoughtSync
+release. Nothing to compile, no toolchain to install.
 
 Installing natively on Arch matters for more than tidiness: pacman pulls
 `webkit2gtk-4.1` itself and the app runs against your system's graphics stack,
@@ -18,14 +18,18 @@ pacman system:
 curl -fsSL https://git.fabledsword.com/bvandeusen/thoughtsync/raw/branch/main/desktop/packaging/install.sh | sh
 ```
 
-Or grab the `.pkg.tar.zst` from the
+Or grab the `.pkg.tar.*` from the
 [latest release](https://git.fabledsword.com/bvandeusen/thoughtsync/releases/latest)
 and install it directly:
 
 ```sh
-sudo pacman -U thoughtsync-desktop-*-x86_64.pkg.tar.zst
+sudo pacman -U thoughtsync-desktop-*-x86_64.pkg.tar.*
 ```
 
+The compression suffix depends on what the build image provides — `.zst` when
+`zstd` is installed, otherwise `.xz` (today's builds are `.xz`). pacman reads
+all of them; only the filename differs.
+
 Either way you get:
 
 - `/usr/bin/thoughtsync` — the app