fix(app): hide server default controls with one server

[Hona]

Hides the server "Set as default" / "Remove default" menu items and the "Default" badge when only one server is registered. The default only affects startup selection and list ordering, both of which are no-ops with a single server, so the control was a confusing toy.

Before	After

defaults.available() is the one gate every menu and badge already reads, so the count check lives there:

// packages/app/src/servers/registry/controller.ts
function useDefaultServer(count: () => number) {
  ...
  available: createMemo(() => !!platform.getDefaultServer && !!platform.setDefaultServer && count() > 1),
}

const defaults = useDefaultServer(() => server.list.length)

Why it's a no-op with one server (the only readers of the stored key):

• sortServerConnections – orders the default first
• availableStartupServer – defaultServer ?? "sidecar", resolves to the only server either way
• ServersProvider.defaultServer – accepted but unused in init

The stored key is retained, so adding a second server later restores "Remove default" on the one that was already chosen.

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