Localize snapshot import/export validation errors

The snapshot envelope validation errors in `src/main/lib/snapshots/exportImport.ts` (`validateExportEnvelope()`) are hardcoded English strings that surface directly in the import UI:

- `Invalid file: not a JSON object`
- `Invalid file: not a Comfy Desktop snapshot export`
- `Unsupported snapshot version: N` (and the newer-file variant added in faf30f39: `This file was created by a newer version of Comfy Desktop; updating the app will likely allow importing it.`)
- `File contains no snapshots`
- `Invalid snapshot at index N` (and the `snapshot version exceeds envelope version` variant)

Other user-facing snapshot strings go through `i18n.t()` with keys in `locales/en.json` / `locales/zh.json`, so non-English users currently get untranslated errors on a failed import.

Follow-up from #1248: move these messages to i18n keys. Note that `validateExportEnvelope()` is also called from contexts where the message is logged rather than shown, so either translate at the throw site (consistent with `snapshotRestoreAfterInstallFailed` usage) or attach a key/params and translate at the UI boundary.

Start in src/main/lib/snapshots/exportImport.ts at validateExportEnvelope() and inspect its callers, especially the import UI and logging contexts. Review the existing snapshot translation keys and snapshotRestoreAfterInstallFailed usage in locales/en.json and locales/zh.json. Done means every listed validation error has i18n coverage with its parameters preserved, while logged errors remain appropriate in non-UI contexts.

Written by the indexing model from the issue text.