fix(release): stamp the Arch package version, and ship the licence with it

pkgver sat at 0.0.18 while the tree was on 0.8.x, because the Arch package is
built by makepkg rather than the tauri bundler and set-version.sh never touched
it. makepkg produced a package whose version bore no relation to the source it
was built from — the exact failure that script exists to prevent, in the one
file it had missed.

Dev versions are converted to a pkgver Arch accepts: a hyphen separates pkgver
from pkgrel, so 0.9.0-3-gabc1234 becomes 0.9.0.r3.gabc1234. pkgrel resets to 1,
since a new upstream version restarts its packaging revisions.

Also installs LICENSE into /usr/share/licenses — MIT is not in Arch's common
licences, so a package under it has to carry the text.

Arch is not part of the automated release (build-release.yml covers linux,
windows and android), so v0.9.0 is unaffected; this applies to anyone building
the package by hand.

packaging/arch/PKGBUILD

@@ -8,7 +8,7 @@
 # tarball/VCS URL and drop the local-copy prepare() step.
 
 pkgname=jellytau
-pkgver=0.0.18
+pkgver=0.9.0
 pkgrel=1
 pkgdesc="A cross-platform Jellyfin client"
 arch=('x86_64')
@@ -42,6 +42,11 @@ package() {
   install -Dm644 "packaging/arch/jellytau.desktop" \
     "$pkgdir/usr/share/applications/jellytau.desktop"
 
+  # MIT is not in /usr/share/licenses/common, so Arch packaging requires the
+  # licence text to ship with the package.
+  install -Dm644 "LICENSE" \
+    "$pkgdir/usr/share/licenses/$pkgname/LICENSE"
+
   # Icons (hicolor)
   install -Dm644 "src-tauri/icons/32x32.png" \
     "$pkgdir/usr/share/icons/hicolor/32x32/apps/jellytau.png"