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/components/session/SessionOutcomeModal.tsx

@@ -6,7 +6,7 @@ import type { SessionOutcome } from '@/types'
 interface SessionOutcomeModalProps {
   isOpen: boolean
   onClose: () => void
-  onSubmit: (data: { outcome: SessionOutcome; outcome_notes?: string }) => Promise<void>
+  onSubmit: (data: { outcome: SessionOutcome; outcome_notes?: string; next_steps?: string }) => Promise<void>
   isSubmitting?: boolean
 }
 
@@ -30,10 +30,12 @@ export function SessionOutcomeModal({
     const formData = new FormData(formRef.current)
     const outcome = (formData.get('session-outcome') as SessionOutcome | null) ?? 'resolved'
     const outcomeNotes = ((formData.get('outcome-notes') as string | null) ?? '').trim()
+    const nextSteps = ((formData.get('next-steps') as string | null) ?? '').trim()
 
     await onSubmit({
       outcome,
       outcome_notes: outcomeNotes || undefined,
+      next_steps: nextSteps || undefined,
     })
   }
 
@@ -113,6 +115,21 @@ export function SessionOutcomeModal({
             )}
           />
         </div>
+
+        <div>
+          <label className="block text-sm font-medium text-white">Next Steps / Follow-Up (optional)</label>
+          <textarea
+            name="next-steps"
+            defaultValue=""
+            rows={3}
+            placeholder="Actions to take after this session..."
+            className={cn(
+              'mt-1 block w-full rounded-md border border-white/10 bg-black/50 px-3 py-2',
+              'text-sm text-white placeholder:text-white/40',
+              'focus:border-white/30 focus:outline-none focus:ring-1 focus:ring-white/20'
+            )}
+          />
+        </div>
       </form>
     </Modal>
   )