ui: bigtrace tidy the topbar and stop hammering the backend

Three finishing changes.

The omnibox spanned the top of every page to offer a command palette
over four commands, three of which already had a button or a hotkey, so
it goes and the palette with it; the theme toggle and the shortcuts
dialog it used to hide move behind one overflow beside the connection
control. Every two-state toggle becomes one welded button group with
both arms outlined, so the border belongs to the pair rather than
sitting on whichever arm happens to be off.

The last is a behaviour fix. Fetching a page of results costs a count
over the whole result table as well as the page, both under the lock the
worker writes through, and three things were asking far more often than
anything needed: the status poll refetched rows every three seconds for
the life of a query, scrolling issued one request per scroll tick, and
nothing was ever cancelled. The poll now carries progress only, requests
wait for the window to settle, and each fetch is aborted by whatever
supersedes it. Measured: a one-second drag went from twenty requests to
one, a flick of the scrollbar from twenty-nine to one.