Name the generator on the C# option, not just its group

[mgravell]

Follow-up to #3, which landed a minute before this was pushed.

There are two C# targets now — protobuf-net's and Google's — and the picker already separates them by group and by value (csharp vs protoc:csharp). That is not enough when the select is closed: browsers render only the option's own text, never its group label, so a collapsed picker read plain "C#" whichever of the two was selected.

The parenthetical goes where the ambiguity is. VB.NET has no protoc counterpart and the rest of the protoc list has no protobuf-net counterpart, so those stay plain.

protobuf-net — in your browser
  C# (protobuf-net)      ← default, unchanged
  VB.NET
protoc — on protoc.protobuf-net.dev
  C++
  C# (Google.Protobuf)
  …

One line of markup; tsc, vitest and vite build are unaffected and pass.

[mgravell]

The group headings already do this work where it matters. Naming the generator on the option too reads as noise everywhere except the collapsed select, which is not worth carrying it for.