Add ability to end a run as victory or defeat

Add End Run button to run dashboard with a confirmation modal offering
Victory/Defeat choice. Backend auto-sets completedAt timestamp and
validates only active runs can be ended. Ended runs show a completion
banner with date and duration, rename team to "Final Team", and hide
encounter logging and status change interactions.

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

frontend/src/components/index.ts

@@ -1,4 +1,5 @@
 export { EncounterModal } from './EncounterModal'
+export { EndRunModal } from './EndRunModal'
 export { GameCard } from './GameCard'
 export { GameGrid } from './GameGrid'
 export { Layout } from './Layout'