Pipeline Update Link Interaction Improvements

This is a followup to #276949 (closed).

Improvement 1

Our current links implementation in the new pipeline graph generates all dependencies on first hover. Let's refactor the code to just generate the dependencies for the hovered link and memoize it, so that we can build up the links bit-by-bit.

⚠️ This might become unnecessary if we expect to switch to only a DAG-driven layout / unify the representation of the graph as all-DAG, because then the links will have to be calculated upfront.

Improvement 2

We show a notice when there are a large number of jobs that might affect the performance. This can probably be designed better — a toggle or something even better.