feat: Add retry mechanism for blocked tasks

Blocked tasks (from spawn failures) were a dead-end with no way to
recover. Add retryBlockedTask to DispatchManager that resets status
to pending and re-queues, a tRPC mutation that also kicks dispatchNext,
and a Retry button in the task slide-over when status is blocked.

apps/server/dispatch/phase-manager.test.ts

@@ -50,6 +50,7 @@ function createMockDispatchManager(): DispatchManager {
     dispatchNext: vi.fn().mockResolvedValue({ success: false, reason: 'mock' }),
     completeTask: vi.fn(),
     blockTask: vi.fn(),
+    retryBlockedTask: vi.fn(),
     getQueueState: vi.fn().mockResolvedValue({ queued: [], ready: [], blocked: [] }),
   };
 }