fix(desktop): recover from corrupt cached Electron download on build

hermes desktop failed on Linux with an ENOENT renaming
release/linux-unpacked/electron -> Hermes. Root cause is a corrupt
cached Electron zip (~/.cache/electron/electron-*.zip): app-builder
unpack-electron extracts a partial tree from the bad zip that is
missing the electron binary, so electron-builder dies on the final
rename. Re-running repeats the broken extraction, leaving the desktop
app permanently unlaunchable until the cache is manually purged.

- Add _electron_download_cache_dirs() + _purge_corrupt_electron_cache()
  to hermes_cli/main.py: validate every electron-*.zip via
  zipfile.testzip() and delete corrupt ones; honor electron_config_cache
  / ELECTRON_CACHE overrides with per-OS defaults.
- Wire purge + single retry into cmd_gui packaged-build failure path so
  a poisoned download self-heals (electron re-downloads clean).
- Add beforePack hook (apps/desktop/scripts/before-pack.cjs) to wipe the
  target unpacked dir before staging, making packaging idempotent across
  interrupted runs. Cross-platform, best-effort.
- Tests: corrupt-zip detector, cmd_gui purge/retry/launch path,
  no-retry-when-clean path, and node --test for the cleanup helper.

apps/desktop/scripts/before-pack.test.cjs

@@ -0,0 +1,53 @@
+const assert = require('node:assert/strict')
+const fs = require('node:fs')
+const os = require('node:os')
+const path = require('node:path')
+const test = require('node:test')
+
+const { cleanStaleAppOutDir } = require('../scripts/before-pack.cjs')
+
+test('cleanStaleAppOutDir removes a populated unpacked directory', () => {
+  const tempRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'hermes-before-pack-'))
+  try {
+    const appOutDir = path.join(tempRoot, 'linux-unpacked')
+    fs.mkdirSync(appOutDir, { recursive: true })
+    // Reproduce the corrupted partial state: license + payload present,
+    // electron binary missing — exactly what trips the ENOENT rename.
+    fs.writeFileSync(path.join(appOutDir, 'LICENSE.electron.txt'), 'x', 'utf8')
+    fs.writeFileSync(path.join(appOutDir, 'resources.pak'), 'x', 'utf8')
+    fs.mkdirSync(path.join(appOutDir, 'resources'), { recursive: true })
+    fs.writeFileSync(path.join(appOutDir, 'resources', 'app.asar'), 'x', 'utf8')
+
+    const removed = cleanStaleAppOutDir(appOutDir)
+
+    assert.equal(removed, true)
+    assert.equal(fs.existsSync(appOutDir), false)
+  } finally {
+    fs.rmSync(tempRoot, { recursive: true, force: true })
+  }
+})
+
+test('cleanStaleAppOutDir is a no-op when the directory is absent', () => {
+  const tempRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'hermes-before-pack-'))
+  try {
+    const missing = path.join(tempRoot, 'does-not-exist')
+    assert.equal(cleanStaleAppOutDir(missing), false)
+  } finally {
+    fs.rmSync(tempRoot, { recursive: true, force: true })
+  }
+})
+
+test('cleanStaleAppOutDir ignores empty or invalid input', () => {
+  assert.equal(cleanStaleAppOutDir(''), false)
+  assert.equal(cleanStaleAppOutDir(undefined), false)
+  assert.equal(cleanStaleAppOutDir(null), false)
+  assert.equal(cleanStaleAppOutDir(42), false)
+})
+
+test('beforePack default export resolves even when cleanup throws', async () => {
+  const { default: beforePack } = require('../scripts/before-pack.cjs')
+  // A directory path that rmSync can't remove is simulated by passing a
+  // context whose appOutDir is a file the hook will try (and be allowed) to
+  // remove; the contract under test is that the hook never rejects.
+  await assert.doesNotReject(beforePack({ appOutDir: '', electronPlatformName: 'linux' }))
+})