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/app/(admin)/admin/rounds/[roundId]/page.tsx

@@ -1265,17 +1265,32 @@ export default function RoundDetailPage() {
                 <div>
                   <p className="text-[11px] font-semibold uppercase tracking-wider text-muted-foreground mb-2">Project Management</p>
                   <div className="grid gap-3 sm:grid-cols-2 lg:grid-cols-3">
-                    <Link href={poolLink}>
-                      <button className="flex items-start gap-3 p-4 rounded-lg border hover:-translate-y-0.5 hover:shadow-md transition-all text-left w-full">
+                    {isMentoring ? (
+                      <button
+                        onClick={() => setActiveTab('projects')}
+                        className="flex items-start gap-3 p-4 rounded-lg border hover:-translate-y-0.5 hover:shadow-md transition-all text-left w-full"
+                      >
                         <Layers className="h-5 w-5 text-[#557f8c] mt-0.5 shrink-0" />
                         <div>
                           <p className="text-sm font-medium">Assign Projects</p>
                           <p className="text-xs text-muted-foreground mt-0.5">
-                            Add projects from the pool to this round
+                            Open the Projects tab to add or auto-fill teams in this round
                           </p>
                         </div>
                       </button>
-                    </Link>
+                    ) : (
+                      <Link href={poolLink}>
+                        <button className="flex items-start gap-3 p-4 rounded-lg border hover:-translate-y-0.5 hover:shadow-md transition-all text-left w-full">
+                          <Layers className="h-5 w-5 text-[#557f8c] mt-0.5 shrink-0" />
+                          <div>
+                            <p className="text-sm font-medium">Assign Projects</p>
+                            <p className="text-xs text-muted-foreground mt-0.5">
+                              Add projects from the pool to this round
+                            </p>
+                          </div>
+                        </button>
+                      </Link>
+                    )}
 
                     <button
                       onClick={() => setActiveTab('projects')}
@@ -1595,7 +1610,12 @@ export default function RoundDetailPage() {
           {isMentoring && (
             <>
               <MentoringBulkAssignToolbar roundId={roundId} configJson={config} />
-              <MentoringProjectsTable roundId={roundId} />
+              <MentoringProjectsTable
+                roundId={roundId}
+                competitionId={competitionId}
+                competitionRounds={competition?.rounds}
+                currentSortOrder={round?.sortOrder}
+              />
             </>
           )}
           {!isMentoring && (