feat(frontend): add next_steps to types, completion modal, and detail page

- Add next_steps to Session, SessionUpdate, SessionComplete, SessionExport types
- Add Next Steps textarea to SessionOutcomeModal
- Update TreeNavigationPage consumer callback for next_steps
- Display next_steps with whitespace-pre-wrap on SessionDetailPage

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/SessionDetailPage.tsx

@@ -332,6 +332,12 @@ export function SessionDetailPage() {
             {session.outcome_notes && (
               <p className="mt-2 text-sm text-white/60">Outcome Notes: {session.outcome_notes}</p>
             )}
+            {session.next_steps && (
+              <div className="mt-2">
+                <span className="text-sm text-white/40">Next Steps:</span>
+                <p className="mt-0.5 text-sm text-white/60 whitespace-pre-wrap">{session.next_steps}</p>
+              </div>
+            )}
           </div>
 
           {/* Actions */}

frontend/src/pages/TreeNavigationPage.tsx

@@ -338,7 +338,7 @@ export function TreeNavigationPage() {
     openCompletionModal(completionDecision, 'standard')
   }
 
-  const handleSubmitOutcome = async (data: { outcome: SessionOutcome; outcome_notes?: string }) => {
+  const handleSubmitOutcome = async (data: { outcome: SessionOutcome; outcome_notes?: string; next_steps?: string }) => {
     if (!session) return
     setIsCompleting(true)
     setError(null)