fix(desktop updates): Linux package installs update via their package manager

Reported on CachyOS: "Check for Updates → The update downloaded but
couldn't be installed … Permission denied (os error 13) at path
/usr/bin/tauri_current_app…". The app was installed from the Arch package;
Tauri's Linux updater can only replace an AppImage.

With cinny-desktop's new update_install_kind command:
- pacman / deb installs: the toast says the update is available and opens
  Settings → General → App Updates, which shows the package-manager command
  (`sudo pacman -U …pkg.tar.zst`, or the .deb + `sudo apt install`) with
  Copy command and Download package — no Install & Restart that can't work.
  "Copied" only when the clipboard write actually succeeded.
- other distros: a link to the downloads page.
- Windows, AppImage, and desktop builds without the command: unchanged
  in-app update.
- a native "package-managed" refusal shows the same help.

Tests: unit (kinds, commands, refusal detection); in the real client with a
simulated desktop bridge: pacman → toast + Settings command/buttons,
install never attempted; older desktop → in-app flow as before. Unit 1321,
Playwright 26 passed.

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

src/app/utils/updateErrors.test.ts

@@ -55,3 +55,35 @@ test('manual download link: Windows gets the installer, others the release page'
   );
   assert.equal(manualDownloadUrl('Mozilla/5.0 (X11; Linux x86_64)'), MANUAL_DOWNLOAD_URL.other);
 });
+
+test('install kinds: anything unknown keeps the in-app updater', async () => {
+  const { toInstallKind } = await import('./updateErrors');
+  assert.equal(toInstallKind('pacman'), 'pacman');
+  assert.equal(toInstallKind('deb'), 'deb');
+  assert.equal(toInstallKind('manual'), 'manual');
+  assert.equal(toInstallKind('in-app'), 'in-app');
+  assert.equal(toInstallKind(undefined), 'in-app', 'older desktop build without the command');
+  assert.equal(toInstallKind('rpm'), 'in-app');
+});
+
+test('package installs get their package manager’s command, not an Install button', async () => {
+  const { packageUpdateHelp, isPackageManagedError } = await import('./updateErrors');
+  const pacman = packageUpdateHelp('pacman')!;
+  assert.equal(
+    pacman.command,
+    'sudo pacman -U https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64.pkg.tar.zst',
+  );
+  assert.match(pacman.url, /LotusChat-x86_64\.pkg\.tar\.zst$/);
+  const deb = packageUpdateHelp('deb')!;
+  assert.match(deb.command!, /sudo apt install \.\/LotusChat-x86_64\.deb$/);
+  assert.equal(packageUpdateHelp('manual')?.command, undefined);
+  assert.equal(packageUpdateHelp('in-app'), undefined);
+  assert.equal(
+    isPackageManagedError('package-managed (pacman): update Lotus Chat with your package manager'),
+    true,
+  );
+  assert.equal(
+    isPackageManagedError('Permission denied (os error 13) at path "/usr/bin/tauri_current_app"'),
+    false,
+  );
+});