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/features/settings/general/General.tsx

@@ -110,7 +110,8 @@ import {
 } from '../../../utils/translation/langUtils';
 import { chromeTranslationEngine } from '../../../utils/translation/chromeEngine';
 import { SequenceCardStyle } from '../styles.css';
-import { useTauriUpdater } from '../../../hooks/useTauriUpdater';
+import { UpdateProgress, useTauriUpdater } from '../../../hooks/useTauriUpdater';
+import { describeUpdateError, manualDownloadUrl } from '../../../utils/updateErrors';
 import { isTauri as isTauriEnv, invokeTauri, tauriInvoke } from '../../../hooks/useTauri';
 import { isSafeGlobalToggleKey } from '../../../hooks/useCallHotkeys';
 import { customWindowChromeAtom } from '../../../state/customWindowChrome';
@@ -2682,6 +2683,22 @@ function Messages() {
   );
 }
 
+const formatMb = (bytes: number): string => `${(bytes / 1_048_576).toFixed(1)} MB`;
+
+function updateProgressText(progress: UpdateProgress | undefined): string {
+  if (!progress) return 'Checking for the update…';
+  if (progress.phase === 'installing') return 'Installing — Lotus Chat will restart in a moment…';
+  if (progress.phase === 'retrying') {
+    return `The update server didn’t respond. Trying again in ${progress.waitSecs} s (attempt ${progress.attempt + 1} of ${progress.maxAttempts})…`;
+  }
+  const attempt =
+    progress.attempt > 1 ? ` (attempt ${progress.attempt} of ${progress.maxAttempts})` : '';
+  const { downloaded, total } = progress;
+  if (!total) return `Downloading update… ${formatMb(downloaded)}${attempt}`;
+  const pct = Math.min(100, Math.floor((downloaded / total) * 100));
+  return `Downloading update… ${pct}% (${formatMb(downloaded)} of ${formatMb(total)})${attempt}`;
+}
+
 function AppUpdates() {
   const { isTauri, status, check, install } = useTauriUpdater();
   if (!isTauri) return null;
@@ -2694,18 +2711,41 @@ function AppUpdates() {
         : status.state === 'available'
           ? `Update available: v${status.version}`
           : status.state === 'installing'
-            ? 'Installing update, the app will restart shortly...'
+            ? updateProgressText(status.progress)
             : status.state === 'error'
-              ? `Update check failed: ${status.message}`
+              ? describeUpdateError(status.phase, status.message)
               : 'Check for a new version of Lotus Chat.';
 
+  const retry = () => {
+    if (status.state === 'error' && status.phase !== 'check') install();
+    else check();
+  };
+
   const after =
     status.state === 'available' ? (
-      <Button size="300" radii="300" onClick={install}>
+      <Button size="300" radii="300" onClick={() => install()}>
         <Text size="B300">Install &amp; Restart</Text>
       </Button>
     ) : status.state === 'checking' || status.state === 'installing' ? (
       <Spinner variant="Secondary" size="200" />
+    ) : status.state === 'error' ? (
+      <Box gap="200" wrap="Wrap" justifyContent="End">
+        <Button size="300" radii="300" variant="Secondary" onClick={retry}>
+          <Text size="B300">Try again</Text>
+        </Button>
+        {status.phase !== 'check' && (
+          <Button
+            size="300"
+            radii="300"
+            variant="Secondary"
+            fill="None"
+            outlined
+            onClick={() => window.open(manualDownloadUrl(navigator.userAgent), '_blank')}
+          >
+            <Text size="B300">Download installer</Text>
+          </Button>
+        )}
+      </Box>
     ) : (
       <Button size="300" radii="300" variant="Secondary" onClick={check}>
         <Text size="B300">Check</Text>
@@ -2717,6 +2757,11 @@ function AppUpdates() {
       <Text size="L400">App Updates</Text>
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile title="Check for Updates" description={description} after={after} />
+        {status.state === 'error' && (
+          <Text size="T200" priority="300" style={{ wordBreak: 'break-word' }}>
+            Details: {status.message}
+          </Text>
+        )}
       </SequenceCard>
     </Box>
   );