The equation of a curve is \(\sqrt{x} + \sqrt{y} = \sqrt{a}\), where \(a\) is a positive constant.

(i) Express \(\frac{dy}{dx}\) in terms of \(x\) and \(y\).

(ii) The straight line with equation \(y = x\) intersects the curve at the point \(P\). Find the equation of the tangent to the curve at \(P\).