network-flow(donut): put the numbers back into the legend

A share tells the reader how the ring is divided, not how much traffic
crossed the wire, and 40% of a total the eye has to hunt for in the middle
of the ring is not a number anyone can act on. So the legend stops
repeating what the arcs already show and states the volume per category
instead - the share is still there, in the center block and in the arc
labels.

That turns the list into a table, and a table needs its header to hold
still while the rows scroll, or a reader who scrolls past the first screen
no longer knows which column is which. The graphing legend splits into a
header table and a body table to get that, which costs it a resize observer
and a padding hack to keep two tables agreeing on their column widths; here
the name column is fluid and the type sizes come off a container query, so
one table with sticky header cells is both shorter and the only version
that resolves those widths once.

Two columns are built but stay out of the way: a category that carries a
previous value gets Prev and a signed change, and until the endpoint
delivers history no row carries one, so the header stays at three columns.
"Other" is not a category but a remainder, so it is the one row drawn with
a chevron - what sits behind it is the breakdown, which nothing consumes
yet.

CMK-37876

Change-Id: Id5775e82c749769c7d33896954ce37453474678c