feat(frontend): find a person on Manage Teams

[sabinem]

Manage Teams: find a person by name

As an organizer arranging a hundred people into teams, I can type part of
someone's name and see at once where they are — on which team, or in
Unassigned — and jump straight to them. Before, the only way was to scan every
team card and scroll through the Unassigned column by eye, or use the browser's
own find, which knows nothing about the Unassigned column's separate scrolling
or about accents.

• A "Find a person…" field sits right above the projects and the Unassigned
  column, and sticks below the save bar while you scroll, so the field and the
  counter stay in view as you jump between matches.
• Everyone whose name matches gets an accent outline, with the matched letters
  marked. The current match gets a thicker outline, and a counter shows where you
  are ("2 of 3").
• Typing jumps to the first match; Enter and ↓ go to the next one, Shift+Enter
  and ↑ to the previous one, wrapping around. Esc or × clears. Jumping scrolls
  the page, and the Unassigned column too when the person is there.
• Matching ignores case and accents and finds a match anywhere in the name:
  "jose" finds "José Núñez", "nunez" too.
• The search only highlights. It never hides or reorders anyone, and it works
  independently of the answer and project filters. Someone in the greyed "Not
  matching" part of Unassigned is shown at full strength when the search finds
  them.

Why the field sticks

Browsers keep the field you are typing in on screen. While the field scrolled
away with the page, every jump to someone further down was pulled straight back
up to it, so people far down the page could not be reached by typing. Keeping
the field in view removes that tug. Its sticking position is measured from the
save bar's actual height, so it stays below the bar when the bar wraps onto two
lines on a narrow screen.

Deliberately left out

• Names only: the search does not look at registration answers or emails.
  Answers already have the filters, and emails are not shown on this page.
• Not remembered: unlike the filters, the search is cleared when you leave
  the page. A search left over from yesterday would look like people had been
  marked for some reason.
• Typing restarts at the first match: each keystroke jumps back to the
  first match rather than staying near the current one.
• Unassigned heading hidden while scrolling, an existing problem: the
  Unassigned column sticks about 1rem from the top of the window, which is
  under the nav bar, so its heading disappears behind the bars when you scroll
  down. The sticky search row covers a little more of it. To be fixed
  separately.