Support Nodes fleet table and map presentation

[ktsaou]

Summary

• make the table group-by control optionally hidden and its width configurable
• add semantic theme colors for metric gaps and Map search highlighting
• extend the public TypeScript theme and table contracts additively
• cover the new behavior with focused table and theme tests

Why

The Nodes fleet views need reusable table-header controls and theme-owned
semantic colors. Keeping these contracts in netdata-ui avoids Cloud-only
styling exceptions and duplicated presentation logic.

User and developer impact

Existing callers keep their current behavior because the new table options have
legacy defaults. Consumers can opt into the narrower fleet presentation
contract without changing existing table payloads or theme usage.

Package release and merge order

• This PR and Charts #225 are
  independent; either may merge first.
• After this PR merges, publish a netdata-ui package version newer than 5.5.5
  containing the Fleet semantic theme tokens and additive Table contracts.
• cloud-frontend #5641
  must not merge until that concrete netdata-ui version and the corresponding
  new Charts version are both published and recorded in the Cloud
  package.json and yarn.lock.

Validation

• focused table-header tests
• yarn test src/theme/index.test.js --runInBand
• integrated netdata-ui → Charts → Cloud production build through the Nodes
  fleet workspace installer