Proposal: Add Simplified Chinese (zh-CN) UI support to aw-webui

Hi ActivityWatch maintainers,

I would like to contribute Simplified Chinese (zh-CN) support for the ActivityWatch web UI.

I checked the current UI and it looks like aw-webui does not yet have a full i18n/localization structure in place, so this may involve two parts:

1. adding a basic internationalization mechanism to the frontend
2. translating the visible UI strings to Simplified Chinese

Before starting, I wanted to confirm a few things:

- Would you welcome a contribution for Simplified Chinese support?
- Is `aw-webui` the correct repository for this work?
- Do you have a preferred approach for introducing i18n in the Vue frontend?
- Would you prefer the work to be split into smaller PRs, for example:
- PR 1: add i18n infrastructure
- PR 2: add zh-CN translations

I am new to open source contribution, but I am willing to do the work and iterate based on feedback.

I think Chinese localization could help lower the barrier for Chinese-speaking users and make ActivityWatch easier to adopt.

Thanks for your time.