Support selecting explicit event fields in category regex rules

[TimeToBuildBob]

Problem

The category editor currently lets users create regex rules, but it does not let them say which event field the regex should apply to.

On the Python stack, the query engine already supports this via select_keys. Exposing that in the web UI would allow narrower rules such as:

{"type": "regex", "regex": "Firefox", "select_keys": ["app"]}

instead of always matching across every string field.

This is useful even without editor-bucket work, because it helps distinguish canonical event fields like app vs title and is closely related to #678.

Scope

• Extend the category rule shape in the web UI to support optional select_keys
• Expose a field selector in the category editor for canonical category event fields
• Make local helpers/tests respect select_keys as well (src/util/classes.ts, related tests)

Important Dependency

This should not be treated as a webui-only feature. aw-server-rust currently ignores select_keys and still matches all string fields, so Rust parity is needed for consistent behavior:

• Support select_keys in regex categorization and tagging rules aw-server-rust#604

Out Of Scope

This does not solve matching by editor project/workspace, because editor bucket data does not currently flow into canonicalEvents(...).

That larger cross-repo design is tracked separately here:

• Support editor-bucket-aware categorization for editor project/workspace rules activitywatch#1305

Related

• Ability to match category by Editor project? #619
• Configurable Title Attribute #678