fix(desktop): the Linux update command verifies the signed SHA256SUMS before installing (cinny-desktop #36)

The pacman / apt update command installed the downloaded package with
no integrity check. Download it with the release's SHA256SUMS.txt and
.minisig into a fresh temp dir, verify the signature with the updater
key (minisign), check the hash, then install.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/updateErrors.test.ts

@@ -69,13 +69,23 @@ test('install kinds: anything unknown keeps the in-app updater', async () => {
 test('package installs get their package manager’s command, not an Install button', async () => {
   const { packageUpdateHelp, isPackageManagedError } = await import('./updateErrors');
   const pacman = packageUpdateHelp('pacman')!;
-  // Not `pacman -U <url>`: that also wants `<url>.sig`, which we don't publish.
-  assert.equal(
-    pacman.command,
-    'curl -LO https://code.lotusguild.org/LotusGuild/cinny-desktop/releases/download/latest/LotusChat-x86_64.pkg.tar.zst && sudo pacman -U ./LotusChat-x86_64.pkg.tar.zst',
+  // [cinny-desktop #36] signature, then checksum, then install, in that order;
+  // a local `pacman -U ./file` (not `<url>`, which also wants `<url>.sig`).
+  const steps = pacman.command!.split(' && ');
+  assert.equal(steps[0], 'cd "$(mktemp -d)"');
+  assert.match(
+    steps[1],
+    /-fLO \S+\/LotusChat-x86_64\.pkg\.tar\.zst -fLO \S+\/SHA256SUMS\.txt -fLO \S+\/SHA256SUMS\.txt\.minisig$/,
   );
+  assert.equal(
+    steps[2],
+    'minisign -Vm SHA256SUMS.txt -P RWRZZdyii05/NUcz337A5SAbiZK+NPVDWuiL2mM5Jk1p/LfRmNfiZ/6l',
+  );
+  assert.equal(steps[3], 'sha256sum -c --ignore-missing SHA256SUMS.txt');
+  assert.equal(steps[4], 'sudo pacman -U ./LotusChat-x86_64.pkg.tar.zst');
   assert.match(pacman.url, /LotusChat-x86_64\.pkg\.tar\.zst$/);
   const deb = packageUpdateHelp('deb')!;
+  assert.match(deb.command!, /minisign -Vm SHA256SUMS\.txt/);
   assert.match(deb.command!, /sudo apt install \.\/LotusChat-x86_64\.deb$/);
   assert.equal(packageUpdateHelp('manual')?.command, undefined);
   assert.equal(packageUpdateHelp('in-app'), undefined);