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/types/session.ts

@@ -57,6 +57,7 @@ export interface Session {
   client_name: string | null
   exported: boolean
   scratchpad: string
+  next_steps: string
 }
 
 export interface SessionCreate {
@@ -72,17 +73,22 @@ export interface SessionUpdate {
   ticket_number?: string
   client_name?: string
   scratchpad?: string
+  next_steps?: string
 }
 
 export interface SessionExport {
   format: 'text' | 'markdown' | 'html' | 'psa'
   include_timestamps?: boolean
   include_tree_info?: boolean
+  include_outcome_notes?: boolean
+  include_next_steps?: boolean
+  max_step_index?: number
 }
 
 export interface SessionComplete {
   outcome: SessionOutcome
   outcome_notes?: string
+  next_steps?: string
 }
 
 // Navigation state for active session