fix: correct SSE connectSSE type cast and explicit url

[Copilot]

connectSSE in ui/src/utils/sse.ts was casting onEvent to (event: unknown) => void, which is incompatible with SSEConfig's required signature, and omitted the url property — breaking the TypeScript build.

Changes:

• Pass url: '/events' explicitly to satisfy SSEConfig
• Fix onEvent cast to (event: { type: string; data: unknown; [key: string]: unknown }) => void matching the actual SSEConfig contract
• Move SSE error suppression from post-hoc es.onerror assignment to SSEConfig.onError

// Before
const es = sharedConnectSSE({ onEvent: onEvent as (event: unknown) => void });
es.onerror = () => { /* suppress */ };

// After
const es = sharedConnectSSE({
  url: '/events',
  onEvent: onEvent as (event: { type: string; data: unknown; [key: string]: unknown }) => void,
  onError: () => { /* suppress */ },
});

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