feat(desktop): update failures say what happened and what to do

A friend's update failed ten times in a row ("Update check failed: error
sending request for url (…nsis.zip)") before the 11th went through, and
he didn't know what to do. The label was also wrong: the check had
worked; the download failed.

- Progress: "Downloading update… 28% (14.3 MB of 49.9 MB)", "The update
  server didn't respond. Trying again in 3 s (attempt 2 of 4)…", from the
  native `lotus-update-progress` events (cinny-desktop retries itself).
- Failures name the step (check / download / install, from the native
  error prefix) in plain language, with Try again and a Download
  installer button (Windows: the setup .exe; else the release page), and
  the raw error under "Details".
- Installing from the update toast now shows a "Downloading update"
  toast, and on failure a sticky "Update didn't install" toast that
  retries on click and points at Settings → General → App Updates.
  Before, the toast vanished and the failure was only visible in Settings.

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

src/app/utils/updateErrors.ts

@@ -0,0 +1,49 @@
+/** Which step of the desktop update failed (the native side prefixes its errors). */
+export type UpdatePhase = 'check' | 'download' | 'install';
+
+/** Where to get the installer by hand when the in-app update keeps failing. */
+export const MANUAL_DOWNLOAD_URL = {
+  windows:
+    'https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64-setup.exe',
+  other: 'https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/tag/latest',
+};
+
+export const manualDownloadUrl = (userAgent: string): string =>
+  /Windows/i.test(userAgent) ? MANUAL_DOWNLOAD_URL.windows : MANUAL_DOWNLOAD_URL.other;
+
+const NETWORK_PATTERN =
+  /error sending request|timed out|timeout|connection|connect|dns|network|reset|refused|unreachable|Could not fetch a valid release/i;
+
+export const isNetworkUpdateError = (message: string): boolean => NETWORK_PATTERN.test(message);
+
+/** Split `download: error sending request …` into its phase and the raw message. */
+export const parseUpdateError = (
+  raw: string,
+  fallback: UpdatePhase,
+): { phase: UpdatePhase; message: string } => {
+  const match = /^(check|download|install): ([\s\S]*)$/.exec(raw);
+  return match
+    ? { phase: match[1] as UpdatePhase, message: match[2] }
+    : { phase: fallback, message: raw };
+};
+
+/**
+ * A plain-language sentence for an update failure. The raw error ("error
+ * sending request for url (…nsis.zip)") told users nothing about what to do.
+ */
+export const describeUpdateError = (phase: UpdatePhase, message: string): string => {
+  const network = isNetworkUpdateError(message);
+  if (phase === 'check') {
+    return network
+      ? 'Couldn’t reach the update server. It may be busy, or your connection dropped. Lotus Chat still works; try again in a minute.'
+      : 'Couldn’t check for updates. Lotus Chat still works; try again later.';
+  }
+  if (phase === 'download') {
+    return network
+      ? 'The update didn’t finish downloading after several tries. The update server may be busy. Try again in a minute, or download the installer yourself.'
+      : /signature|minisign|base64/i.test(message)
+        ? 'The downloaded update didn’t pass its safety check, so it wasn’t installed. Try again, or download the installer yourself.'
+        : 'The update couldn’t be downloaded. Try again, or download the installer yourself.';
+  }
+  return 'The update downloaded but couldn’t be installed. Download the installer yourself and run it; your chats and settings are kept.';
+};