fix(dashboard): thread finishedAt through payload validator + test fixture

CI typecheck caught a DashboardCard fixture in dashboard-payload-validation.test.ts
missing the new finishedAt field; also validate finishedAt in the relay guard.

src/main/ipc/dashboard-payload-validation.test.ts

@@ -21,6 +21,7 @@ const SNAPSHOT = {
       repoName: 'Orca',
       worktreeName: 'Dashboard',
       startedAt: 1_699_999_000_000,
+      finishedAt: null,
       stateChangedAt: 1_699_999_500_000,
       unseen: true,
       askSummary: '{"question":"Proceed?"}'

src/main/ipc/dashboard-payload-validation.ts

@@ -77,6 +77,7 @@ function isDashboardCard(value: unknown): boolean {
     isBoundedString(card.repoName, MAX_LABEL_LENGTH, true) &&
     isBoundedString(card.worktreeName, MAX_LABEL_LENGTH, true) &&
     isFiniteNumber(card.startedAt) &&
+    (card.finishedAt === null || isFiniteNumber(card.finishedAt)) &&
     isFiniteNumber(card.stateChangedAt) &&
     typeof card.unseen === 'boolean' &&
     isOptionalBoundedString(card.askSummary, AGENT_STATUS_INTERACTIVE_PROMPT_MAX_LENGTH)