feat: implement bl user icon (GET /api/v2/users/{userId}/icon)

[23prime]

Type

feat

Summary

Implement bl user icon <userId> command to download the user icon image.

Changes

• Add GET /api/v2/users/{userId}/icon support in src/api/user.rs
• Add bl user icon <userId> --output <path> subcommand in src/cmd/user/icon.rs
• Update docs: website/docs/commands.md, website/i18n/ja/...

Notes

Returns binary image data. Use --output to specify the save path.