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.test.ts

@@ -0,0 +1,57 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  describeUpdateError,
+  manualDownloadUrl,
+  MANUAL_DOWNLOAD_URL,
+  parseUpdateError,
+} from './updateErrors';
+
+test('parseUpdateError reads the native phase prefix', () => {
+  assert.deepEqual(parseUpdateError('download: error sending request for url (x)', 'check'), {
+    phase: 'download',
+    message: 'error sending request for url (x)',
+  });
+  assert.deepEqual(parseUpdateError('install: access denied', 'download'), {
+    phase: 'install',
+    message: 'access denied',
+  });
+});
+
+test('parseUpdateError falls back to the caller phase for unprefixed errors', () => {
+  assert.deepEqual(parseUpdateError('error sending request', 'check'), {
+    phase: 'check',
+    message: 'error sending request',
+  });
+});
+
+test('the reported download failure reads as a busy server, not a failed check', () => {
+  const { phase, message } = parseUpdateError(
+    'download: error sending request for url (https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64-setup.nsis.zip)',
+    'check',
+  );
+  const text = describeUpdateError(phase, message);
+  assert.match(text, /didn’t finish downloading/);
+  assert.match(text, /download the installer yourself/);
+});
+
+test('only signature failures are reported as a failed safety check', () => {
+  assert.match(
+    describeUpdateError('download', 'The signature X could not be decoded'),
+    /safety check/,
+  );
+  assert.doesNotMatch(describeUpdateError('download', 'No space left on device'), /safety/);
+});
+
+test('check failures never suggest the app is broken', () => {
+  assert.match(describeUpdateError('check', 'operation timed out'), /Lotus Chat still works/);
+  assert.match(describeUpdateError('check', 'weird'), /Lotus Chat still works/);
+});
+
+test('manual download link: Windows gets the installer, others the release page', () => {
+  assert.equal(
+    manualDownloadUrl('Mozilla/5.0 (Windows NT 10.0; Win64; x64) Edg/140'),
+    MANUAL_DOWNLOAD_URL.windows,
+  );
+  assert.equal(manualDownloadUrl('Mozilla/5.0 (X11; Linux x86_64)'), MANUAL_DOWNLOAD_URL.other);
+});