Expand the UNS Hierarchy by default & improve hover behaviour

[joepavitt]

Description

It's annoying to have to expand many layers each time, and given that it's a single single to minimise a whole branch of the hierarchy, expanded by default makes more sense.

The default "closed" view also implies there is nothing on the screen, especially if the first nested topic is /

Also improves the hover state, with a pointer:cursor and highlighted text:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.69%. Comparing base (7eebf0f) to head (e0633b9).
Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4854   +/-   ##
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  Coverage   78.69%   78.69%           
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  Files         315      315           
  Lines       15147    15147           
  Branches     3486     3486           
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  Hits        11920    11920           
  Misses       3227     3227           

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[cstns]

The fact that the ff-accordion component was rendering the html elements was the main reason i ended up not using it for rendering the nested hierarchy. For small payloads with <50 topics it behaves nicely but when you exceed that, the initial page render and DOM draw over cumbers the page and might freeze it.

By programmatically hiding the elements and only drawing the starting segments I managed to load 1500 entries with >50 nested topics.

I'd go with a variant of opening only the first n topics up to an m level

[joepavitt]

I'd go with a variant of opening only the first n topics up to an m level

That's a fair counter proposal - could I ask you to jump in and cover/extend this PR please? I need to sort other bits before my demo at 12

[cstns]

accepting changes due to the current low number of broker users and topics.

A cohesive implementation will be done in #4857