fix(desktop updates): pacman command downloads first, installs the local file

`sudo pacman -U <url>` also fetches `<url>.sig` and failed (404) on
CachyOS: remote packages fall under RemoteFileSigLevel (signature
required) and we don't sign the package. A downloaded file installs under
LocalFileSigLevel (optional): `curl -LO <url> && sudo pacman -U ./…`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/updateErrors.test.ts

@@ -69,9 +69,10 @@ test('install kinds: anything unknown keeps the in-app updater', async () => {
 test('package installs get their package manager’s command, not an Install button', async () => {
   const { packageUpdateHelp, isPackageManagedError } = await import('./updateErrors');
   const pacman = packageUpdateHelp('pacman')!;
+  // Not `pacman -U <url>`: that also wants `<url>.sig`, which we don't publish.
   assert.equal(
     pacman.command,
-    'sudo pacman -U https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64.pkg.tar.zst',
+    'curl -LO https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64.pkg.tar.zst && sudo pacman -U ./LotusChat-x86_64.pkg.tar.zst',
   );
   assert.match(pacman.url, /LotusChat-x86_64\.pkg\.tar\.zst$/);
   const deb = packageUpdateHelp('deb')!;

src/app/utils/updateErrors.ts

@@ -67,7 +67,14 @@ export type PackageUpdateHelp = { command?: string; url: string; download: strin
 export const packageUpdateHelp = (kind: InstallKind): PackageUpdateHelp | undefined => {
   if (kind === 'pacman') {
     const url = `${RELEASE_DOWNLOAD}/LotusChat-x86_64.pkg.tar.zst`;
-    return { command: `sudo pacman -U ${url}`, url, download: 'Download package' };
+    // Download first, then install the local file: `pacman -U <url>` also
+    // fetches `<url>.sig` and fails without it (we don't sign packages),
+    // while a local file falls under LocalFileSigLevel (signature optional).
+    return {
+      command: `curl -LO ${url} && sudo pacman -U ./LotusChat-x86_64.pkg.tar.zst`,
+      url,
+      download: 'Download package',
+    };
   }
   if (kind === 'deb') {
     const url = `${RELEASE_DOWNLOAD}/LotusChat-x86_64.deb`;