feat: browser-side WebSocket error boundaries and reconnection

[jamilagibson]

Context

Auto-reconnect with exponential backoff is implemented on the RN agent side. The browser UI currently shows a connection status indicator but does not attempt to reconnect if the WebSocket drops mid-session.

Acceptance criteria

• Browser UI attempts reconnection with exponential backoff when the WebSocket closes unexpectedly
• Connection status indicator reflects reconnecting state (distinct from disconnected)
• Snapshot history is preserved across reconnections (not cleared on disconnect)
• React error boundaries prevent a WebSocket failure from crashing the UI