feat: close sessions from history page with inline popover

Add ability to close active sessions directly from the Session History
page via an inline popover with outcome selection and optional notes.
Adds two new outcomes: cancelled and resolved_externally.

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

frontend/src/types/session.ts

@@ -1,7 +1,7 @@
 import type { TreeStructure } from './tree'
 import type { Step, StepContent } from './step'
 
-export type SessionOutcome = 'resolved' | 'escalated' | 'workaround' | 'unresolved'
+export type SessionOutcome = 'resolved' | 'escalated' | 'workaround' | 'unresolved' | 'cancelled' | 'resolved_externally'
 
 export interface DecisionRecord {
   node_id: string