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/package.json

@@ -146,6 +146,7 @@
       "package.json"
     ],
     "beforeBuild": "scripts/before-build.cjs",
+    "beforePack": "scripts/before-pack.cjs",
     "afterPack": "scripts/after-pack.cjs",
     "extraResources": [
       {

apps/desktop/scripts/before-pack.cjs

@@ -0,0 +1,78 @@
+'use strict'
+
+/**
+ * before-pack.cjs — electron-builder beforePack hook.
+ *
+ * Removes any stale unpacked app directory (`appOutDir`) before
+ * electron-builder stages the Electron binaries into it.
+ *
+ * WHY THIS EXISTS
+ * ---------------
+ * electron-builder's final packaging step copies the stock `electron`
+ * binary into `release/<platform>-unpacked/` and then renames it to the
+ * product name (`Hermes`). If a PREVIOUS `npm run pack` was interrupted
+ * (Ctrl-C, OOM kill, crash, full disk) the unpacked directory is left in a
+ * corrupted partial state: it keeps the already-renamed `LICENSE.electron.txt`
+ * and the Chromium payload (.pak/.so/icudtl.dat/chrome-sandbox) but is MISSING
+ * the `electron` binary itself.
+ *
+ * On the next run, electron-builder sees the destination directory already
+ * populated, skips re-copying the binary it thinks is present, then tries to
+ * rename a `electron` file that no longer exists. The build dies with:
+ *
+ *   ENOENT: no such file or directory, rename
+ *   '.../release/linux-unpacked/electron' -> '.../release/linux-unpacked/Hermes'
+ *
+ * This is a hard failure with no obvious cause for the user — `hermes desktop`
+ * just prints "Desktop GUI build failed" and the only fix is to manually
+ * `rm -rf` the release directory, which a normal user has no way to know.
+ *
+ * The packaging step is not idempotent across an interrupted run, so we make
+ * it idempotent ourselves: wipe the target unpacked directory up front so
+ * electron-builder always stages into a clean tree. This is safe — the
+ * directory is a pure build artifact that electron-builder fully recreates
+ * on every pack; nothing else depends on its prior contents.
+ *
+ * Cross-platform: the same partial-state trap exists on macOS
+ * (the mac-unpacked Hermes.app bundle) and Windows (win-unpacked), so we
+ * clean whatever `appOutDir` electron-builder hands us regardless of platform.
+ *
+ * Best-effort: a cleanup failure must never mask the real build. We log and
+ * resolve rather than throw — worst case electron-builder hits the original
+ * ENOENT, which is no worse than not having this hook at all.
+ *
+ * electron-builder passes a context with:
+ *   - appOutDir:            the unpacked app directory about to be staged
+ *   - electronPlatformName: 'win32' | 'darwin' | 'linux'
+ */
+
+const fs = require('node:fs')
+
+function cleanStaleAppOutDir(appOutDir) {
+  if (!appOutDir || typeof appOutDir !== 'string') {
+    return false
+  }
+  if (!fs.existsSync(appOutDir)) {
+    return false
+  }
+  // Recursive + force so a half-written tree (read-only bits, partial files)
+  // can't block the wipe. retry/maxRetries rides out transient EBUSY on
+  // Windows where an AV/indexer may briefly hold a handle.
+  fs.rmSync(appOutDir, { recursive: true, force: true, maxRetries: 5, retryDelay: 100 })
+  return true
+}
+
+exports.cleanStaleAppOutDir = cleanStaleAppOutDir
+
+exports.default = async function beforePack(context) {
+  const appOutDir = context && context.appOutDir
+  try {
+    if (cleanStaleAppOutDir(appOutDir)) {
+      console.log(`[before-pack] removed stale unpacked dir before staging: ${appOutDir}`)
+    }
+  } catch (err) {
+    // Never fail the build over cleanup; surface why so a genuinely stuck
+    // directory (permissions, mount) is still diagnosable.
+    console.warn(`[before-pack] could not clean ${appOutDir} (${err.message}); continuing`)
+  }
+}

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' }))
+})