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.ts

@@ -47,3 +47,39 @@ export const describeUpdateError = (phase: UpdatePhase, message: string): string
   }
   return 'The update downloaded but couldn’t be installed. Download the installer yourself and run it; your chats and settings are kept.';
 };
+
+/**
+ * How this desktop install gets updated (cinny-desktop `update_install_kind`).
+ * Linux package installs can't be replaced in place by the in-app updater
+ * (it tried to write into /usr/bin: "Permission denied (os error 13)"), so
+ * they get their package manager's command instead of an Install button.
+ */
+export type InstallKind = 'in-app' | 'pacman' | 'deb' | 'manual';
+
+const RELEASE_DOWNLOAD =
+  'https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest';
+
+export const toInstallKind = (value: unknown): InstallKind =>
+  value === 'pacman' || value === 'deb' || value === 'manual' ? value : 'in-app';
+
+export type PackageUpdateHelp = { command?: string; url: string; download: string };
+
+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 ${url}`, url, download: 'Download package' };
+  }
+  if (kind === 'deb') {
+    const url = `${RELEASE_DOWNLOAD}/LotusChat-x86_64.deb`;
+    return {
+      command: `curl -LO ${url} && sudo apt install ./LotusChat-x86_64.deb`,
+      url,
+      download: 'Download package',
+    };
+  }
+  if (kind === 'manual') return { url: MANUAL_DOWNLOAD_URL.other, download: 'Open downloads' };
+  return undefined;
+};
+
+/** The native side refuses an in-app install on a package install. */
+export const isPackageManagedError = (message: string): boolean => /package-managed/.test(message);