v0.4.0

Minor release — the i18n + packaging cycle.

Features:
- Full i18n (zh-CN / en) with in-app language switcher; all UI strings
  and format helpers localized.
- Web UI: query latency in status bar, entrance animations, header
  redesign with brand linking to the instance.
- Packaging: .deb build via nfpm (Taskfile deb target).

Docs:
- Bilingual docs (docs/), modernized README, LICENSE added.

No engine changes; --public-base-url and all v0.3.x behavior preserved.