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": [
       {