Query features missing

Grammar:
- [x] support empty statements

Functions:
- [x] find_bucket
- [x] filter by host functionality (in #242)
- [x] adjacent, union, period_union (https://github.com/ActivityWatch/aw-core/pull/65)
- [x] exclude_keyvals (#243)

~Some of these are needed to support the default query in the query explorer in the web-ui~
Since "support empty statements" and "find_bucket" were fixed a few months ago the default query in the query explorer works fine

The issue lists query grammar and functions, but every item is checked and it says the default query already works. No files, tests, or entry points are named; first confirm whether any work remains before identifying an implementation target. Done would require a newly specified missing query behavior.

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