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/mentoring-projects-table.tsx

@@ -33,12 +33,32 @@ import {
   Loader2,
   Download,
   X,
+  Plus,
 } from 'lucide-react'
 import { CountryDisplay } from '@/components/shared/country-display'
+import { AddProjectDialog } from '@/components/admin/round/project-states-table'
 
 type Filter = 'all' | 'unassigned' | 'assigned' | 'wants_only'
 
-export function MentoringProjectsTable({ roundId }: { roundId: string }) {
+type CompetitionRound = {
+  id: string
+  name: string
+  sortOrder: number
+  _count: { projectRoundStates: number }
+}
+
+export function MentoringProjectsTable({
+  roundId,
+  competitionId,
+  competitionRounds,
+  currentSortOrder,
+}: {
+  roundId: string
+  competitionId: string
+  competitionRounds?: CompetitionRound[]
+  currentSortOrder?: number
+}) {
+  const [addProjectOpen, setAddProjectOpen] = useState(false)
   const [search, setSearch] = useState('')
   const [filter, setFilter] = useState<Filter>('all')
   const [selected, setSelected] = useState<Set<string>>(new Set())
@@ -67,6 +87,10 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
         toast.info(
           `No new assignments — every selected mentor is already on every selected project (${result.totalSkipped} pair${result.totalSkipped === 1 ? '' : 's'} skipped).`,
         )
+      } else if (result.totalAssigned === 0 && result.ineligibleProjectCount > 0) {
+        toast.warning(
+          `${result.ineligibleProjectCount} project${result.ineligibleProjectCount === 1 ? '' : 's'} aren't in a mentoring round and were skipped.`,
+        )
       } else {
         const mentorCount = result.perMentor.filter((m) => m.assigned > 0).length
         toast.success(
@@ -197,18 +221,32 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
     return (
       <div className="space-y-3">
         {importBanner}
-        <div className="rounded-md border bg-muted/30 px-4 py-12 text-center text-sm text-muted-foreground">
-          No projects in this mentoring round yet.
-          {!importBanner && (
-            <>
-              {' '}Use{' '}
-              <span className="font-medium text-foreground">
-                Add Project to Round
-              </span>{' '}
-              to populate it.
-            </>
-          )}
+        <div className="flex items-center justify-end">
+          <Button size="sm" onClick={() => setAddProjectOpen(true)}>
+            <Plus className="mr-1.5 h-4 w-4" />
+            Add Project to Round
+          </Button>
         </div>
+        <div className="rounded-md border bg-muted/30 px-4 py-12 text-center text-sm text-muted-foreground">
+          No projects in this mentoring round yet. Click{' '}
+          <span className="font-medium text-foreground">Add Project to Round</span>{' '}
+          above to populate it.
+        </div>
+
+        <AddProjectDialog
+          open={addProjectOpen}
+          onOpenChange={setAddProjectOpen}
+          roundId={roundId}
+          competitionId={competitionId}
+          competitionRounds={competitionRounds}
+          currentSortOrder={currentSortOrder}
+          onAssigned={() => {
+            utils.round.listMentoringProjects.invalidate({ roundId })
+            utils.round.getProjectsNeedingMentor.invalidate({ roundId })
+            utils.round.getMentoringImportCandidates.invalidate({ roundId })
+            utils.mentor.getRoundStats.invalidate({ roundId })
+          }}
+        />
       </div>
     )
   }
@@ -246,14 +284,24 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
           <Pill value="assigned" label="Has mentor" count={totals.assigned} />
           <Pill value="wants_only" label="Wants mentorship" count={totals.wants} />
         </div>
-        <div className="relative w-full sm:max-w-xs">
-          <Search className="absolute left-2.5 top-1/2 h-3.5 w-3.5 -translate-y-1/2 text-muted-foreground" />
-          <Input
-            value={search}
-            onChange={(e) => setSearch(e.target.value)}
-            placeholder="Search projects, teams, or mentors…"
-            className="pl-8"
-          />
+        <div className="flex items-center gap-2">
+          <div className="relative w-full sm:w-72">
+            <Search className="absolute left-2.5 top-1/2 h-3.5 w-3.5 -translate-y-1/2 text-muted-foreground" />
+            <Input
+              value={search}
+              onChange={(e) => setSearch(e.target.value)}
+              placeholder="Search projects, teams, or mentors…"
+              className="pl-8"
+            />
+          </div>
+          <Button
+            size="sm"
+            onClick={() => setAddProjectOpen(true)}
+            className="shrink-0"
+          >
+            <Plus className="mr-1 h-4 w-4" />
+            Add
+          </Button>
         </div>
       </div>
 
@@ -670,6 +718,21 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
           </DialogFooter>
         </DialogContent>
       </Dialog>
+
+      <AddProjectDialog
+        open={addProjectOpen}
+        onOpenChange={setAddProjectOpen}
+        roundId={roundId}
+        competitionId={competitionId}
+        competitionRounds={competitionRounds}
+        currentSortOrder={currentSortOrder}
+        onAssigned={() => {
+          utils.round.listMentoringProjects.invalidate({ roundId })
+          utils.round.getProjectsNeedingMentor.invalidate({ roundId })
+          utils.round.getMentoringImportCandidates.invalidate({ roundId })
+          utils.mentor.getRoundStats.invalidate({ roundId })
+        }}
+      />
     </div>
   )
 }

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,