Fix timeline line width break when size="lg" is set on individual item

[joshhanley]

The Scenario

When using size="lg" on an individual flux:timeline.item, the timeline line becomes thicker (2px) at that item while surrounding items remain at 1px, creating a visible break in the line.

<flux:timeline>
    <flux:timeline.item>
        <flux:timeline.indicator>
            <flux:icon.tag variant="micro" />
        </flux:timeline.indicator>
        <flux:timeline.content>
            <flux:heading>Default item</flux:heading>
        </flux:timeline.content>
    </flux:timeline.item>

    <flux:timeline.item size="lg">
        <flux:timeline.indicator>
            <flux:icon.star variant="micro" />
        </flux:timeline.indicator>
        <flux:timeline.content>
            <flux:heading>Large item</flux:heading>
        </flux:timeline.content>
    </flux:timeline.item>
</flux:timeline>

The Problem

The CSS rule :where([data-flux-timeline-size="lg"]) sets --flux-timeline-line-width: 2px and --flux-timeline-line-gap: 0px. This selector matches any element with the attribute, including individual <li> items. When only one item has size="lg", its line segments inherit the thicker width while surrounding items stay at 1px.

The Solution

Split the CSS rule into two parts:

• Line-related variables (--flux-timeline-line-width, --flux-timeline-line-gap) and timeline-level spacing (--flux-timeline-item-gap, --flux-timeline-content-gap) now use the more specific selector :where([data-flux-timeline][data-flux-timeline-size="lg"]), which only matches the <ol> wrapper.
• The indicator size (--flux-timeline-indicator-size) remains on the broader :where([data-flux-timeline-size="lg"]) selector so it can still scale per-item.

Note: You will see in the fixed screenshot above the different line colours, that will be fixed in a Flux Pro PR https://github.com/livewire/flux-pro/pull/473.

Fixes #2473