Add OnConnectionChangeFunc to WASM UIImpl to fix compilation

[Copilot]

The OnConnectionChangeFunc callback field was added to the non-WASM UIImpl struct but missing from the WASM version, causing compilation failures for WASM builds since both share the same BroflakeOptions struct.

Changes

• Add OnConnectionChangeFunc field to WASM UIImpl struct in ui_wasm_impl.go
• Update OnConsumerConnectionChange method to invoke callback before firing JavaScript event

Both implementations now have consistent callback support:

func (ui UIImpl) OnConsumerConnectionChange(state int, workerIdx int, addr net.IP) {
    if ui.OnConnectionChangeFunc != nil {
        ui.OnConnectionChangeFunc(state, workerIdx, addr)
    }
    // ... WASM-specific event firing continues
}

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