feat: add bonding curve info tooltip to KeySupplyBadge (#381)

[temisan0x]

Closes #381

• Adds an Info icon button next to the supply number in KeySupplyBadge
• Hovering or focusing the icon shows a tooltip explaining key supply and the bonding curve relationship
• Tooltip uses the existing accessible Tooltip component (role="tooltip", aria-describedby, keyboard focus support)
• Info button has aria-label="What is key supply?" for screen readers
• 3 new tests added covering: info button presence, tooltip content, and null supply state
• No existing behaviour changed

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