fix: scale MemberRing with SVG viewBox

[khanavi272-spec]

What changed

• Replace the MemberRing pixel-based layout with an SVG viewBox.
• Let CSS control the SVG size instead of calculating dimensions in JavaScript.
• Preserve the existing member, funded, recipient, and revealed states.
• Preserve the existing accessibility label and ring caption.

Why

The MemberRing previously relied on resize-driven JavaScript positioning. Using a fixed SVG viewBox allows the ring to scale naturally with its container without requiring a window resize listener.

Issue

Closes #305

Testing

• Verified the MemberRing uses a fixed SVG viewBox
• Verified the SVG uses width="100%"
• Verified the resize listener is not used by MemberRing
• Circuit tests
• Contract tests
• E2E against testnet
• App tested manually

Screenshots

Not included because this change only updates the SVG scaling implementation.