feat: SSE disconnect/reconnecting indicator

[uditdc]

Closes #11

api/events.ts had no onerror handler on the EventSource, so a dropped SSE connection was silent to the user. useLiveUpdates now tracks connection state via the EventSource's open/error events and returns it; RepoView surfaces that state as a small amber "Reconnecting…" badge in the top bar whenever the feed drops, using the browser's built-in auto-reconnect (no state is shown while connected).

Acceptance criteria

• Killing the server shows a visible "reconnecting" state in the UI — the error event handler sets connection state to reconnecting, rendering the badge in RepoView's top action bar; verified by reading the EventSource event flow (browsers fire error on a dropped connection and keep auto-retrying until open fires again).
• Reconnecting clears the indicator automatically — the open event handler resets state back to connected, which hides the badge; this fires both on initial connect and on every successful auto-reconnect.

pnpm typecheck, pnpm test, and pnpm build all pass.

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