feat(frontend): rebuild team management as a save-once workspace

[sabinem]

The manage-teams page could not cope with a hundred participants: people were chips in a wrap, preferences were truncated titles, and every drag was a server round trip.

It is now two columns — projects with their teams on the left, unassigned on the right, both scrollable — and people are full-width rows wherever they sit. Each project carries a number, and a participant's preferences show as those numbers rather than titles, with the matching one highlighted inside a team so it is obvious whether someone got what they asked for.

"Suggest teams" builds a whole distribution from everyone's preferences. The rule is deliberately singular: everyone goes to a project they picked, spread evenly across their options, and no team exceeds six. There is no minimum team size. A minimum sounds reasonable and generates every hard case — projects dissolved, people redistributed, totals that satisfy no split, teams emptied when the number is raised — for a judgement that belongs to the organizer anyway. A project two people want is a team of two, and dragging is the tool for deciding otherwise. The algorithm lives in $lib/utils/teamDistribution so it can be tested without a page around it.

The page is now a workspace rather than a series of instructions: dragging, adding, renaming, deleting and clearing all happen in the browser, the header shows what is pending, and one Save writes it. That collapses move, createTeam, renameTeam, deleteTeam and clearAssignments into a single save action which takes the complete desired state and reconciles — deletes first, then creates and renames, then the membership difference.

Applying is slow by construction: there is no bulk RPC, so a hundred people is a few hundred sequential calls.