fix: ensure panel scrollbars appear on Firefox (Bounty #8)

[zp6]

Fix: Firefox scrollbar visibility

Closes #8

Problem

Panel scrollbars do not appear on Firefox, making it impossible to scroll through content.

Solution

• Added Firefox-specific CSS using @supports (-moz-appearance: none)
• Sets scrollbar-width: thin and scrollbar-color for Firefox
• Forces overflow-y: scroll to ensure scrollbar space is reserved
• Also includes webkit scrollbar styles for consistency

Acceptance

• Scrollbars visible on Firefox
• Thin, styled scrollbars that match the design
• No impact on Chrome/Edge behavior

Wallet: zp6