Add expandable media project layer

[sobomax]

What changed

• reveals a second media layer when the RTPProxy card or its spoke is hovered or focused
• links sippy/rtp.io, sippy/rtp_cluster, and sippy/go-rtp_cluster
• adds a wider interactive hit area and highlighted state for the media spoke
• keeps the flyout open while navigating its cards
• renders the related projects inline on tablet and mobile where hover is unavailable

Accessibility

• the media spoke is keyboard-focusable
• every subproject is a normal external link with visible focus styling
• reduced-motion preferences continue to disable animation

Validation

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