fix(mentor): restore Add Project on mentoring rounds + gate mentor assignment

Three related bugs around the mentoring-round Projects tab:

1. Add Project to Round was unreachable on MENTORING rounds — the table swap
   in the prior commit lost the button. Export AddProjectDialog from
   project-states-table and render it inside MentoringProjectsTable with an
   "Add" button in the filter row and a CTA in the empty state.
2. The "Assign Projects" quick action on the round overview linked to the
   global pool with an opaque filter; on MENTORING rounds it now switches
   to the Projects tab where the new Add Project button + auto-fill +
   per-team picker all live. Non-mentoring rounds keep the old behavior.
3. mentor.assign and mentor.bulkAssign now refuse projects that aren't
   enrolled in any MENTORING round (any status). The single-assign throws
   BAD_REQUEST with a guidance message; the bulk path filters them out and
   reports ineligibleProjectCount in the result so the UI can warn the
   admin instead of silently skipping.

Tests: the multi-mentor-assignment suite now sets up a MENTORING round +
ProjectRoundState for each project it tests against, matching the new gate.

src/components/admin/round/project-states-table.tsx

@@ -785,7 +785,7 @@ function QuickAddDialog({
  * Create New: form to create a project and assign it directly to the round.
  * From Pool: search existing projects not yet in this round and assign them.
  */
-function AddProjectDialog({
+export function AddProjectDialog({
   open,
   onOpenChange,
   roundId,