feat(modpack): add self-contained full pack export/import for offline play

[Origin173]

Summary

• Add a new Full Pack (Offline) export format that bundles the version-isolated instance content plus the game libraries (libraries/) and assets (assets/) into a single zip, identified by an xmcl-full-pack.json manifest.
• Import is a pure extraction — nothing is downloaded — so the instance can be launched offline immediately after import.
• Adds two backend commands (scan_instance_files_for_full_export, import_full_pack), a Full format in the export modal, and full-pack-aware import handling (name is fixed by the pack, no download tasks, direct navigation to the instance list).
• Fills the missing exportModpack / scanInstanceFilesForExport / FileTreeSelector.empty service-message translations in fr/ja/zh-Hant so the locale consistency check passes.

Pack layout

<zip>/
├── xmcl-full-pack.json   # { formatVersion:1, name, version, minecraftVersion, modLoader, ... }
├── instance/<name>/      # version-isolated instance content (user selection + mandatory core files)
├── libraries/            # game dir shared libraries
└── assets/               # game dir shared assets (indexes + objects)

Notes

• Already-compressed files (jar/zip/images/audio) are stored as-is; other files are deflated.
• Import validates every entry against path escape (zip-slip protection) and rewrites the instance id to the target game directory.
• Known pre-existing (unchanged): MultiMC packs are still reported with modpackType = "modrinth" in modpack meta info — out of scope for this PR.

Test plan

• cargo check
• npx tsc --noEmit
• npm run locale diff en
• Export an instance as Full Pack and inspect the zip layout + manifest
• Import the pack into a fresh game directory and launch offline