Fix field spacing when only using description

[joshhanley]

The Scenario

When rendering Flux input fields, a description without a label sits directly against the input. The spacing is correct for a label-only field, and also for a field that has both a label and description.

<div class="space-y-8">
    <div>
        <flux:heading size="lg" level="2" class="mb-4">Label</flux:heading>

        <flux:input label="Input label" placeholder="Input value" />
    </div>

    <div>
        <flux:heading size="lg" level="2" class="mb-4">Label + description</flux:heading>

        <flux:input label="Input label" description="Input description" placeholder="Input value" />
    </div>

    <div>
        <flux:heading size="lg" level="2" class="mb-4">Description</flux:heading>

        <flux:input description="Input description" placeholder="Input value" />
    </div>
</div>

The Problem

The field spacing selector handles label+description but not description on it's down. As such the description has no bottom margin when there is no label.

The Solution

Add a field spacing selector for descriptions that are the first child and have following content. This gives description-only fields the same mb-3 gap used between label descriptions and controls.

'[&>[data-flux-description]:first-child:has(+*)]:mb-3',

The selector is scoped to that leading-description case, so existing label + description spacing remains unchanged.

Fixes #2664