Query Caching

I've thought about adding proper caching of queries to make browsing historical data much snappier, after the first open.

The goal is to:
- make the day/week/month selector (simple barchart at top) load faster
- make the "Timeline (Barchart)" load faster

As a first step, I've considered adding query caching to aw-client-js such that queries are:

- cached by timespan
- if timespan spans the future, don't cache
- only cached if the query returned data for the timespan

Should be a simple improvement, that could massively improve responsiveness and thus general UX of web UI, especially when browsing historical data.

I'm not sure what kind of cache expiry to add, but I might figure that out as I get going.

PRs:
- https://github.com/ActivityWatch/aw-client-js/pull/44
- https://github.com/ActivityWatch/aw-webui/pull/501