fix(hands): correct trader dashboard style

[lc-soft]

Summary

Fixed two styling issues in the Hands page of the trader dashboard:

• Content overlap: The absolutely positioned .trader-chart-empty element was incorrectly positioned relative to trader-dashboard, blocking window content and preventing scrolling.
• Transparent background: The window background appeared transparent due to an undefined --bg-card CSS variable.

Changes

Before	After

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

[jaberjaber23]

CSS token rename (--bg-card → --surface) + position: relative on .trader-chart — all reasonable. The current CI failures on this PR are pre-existing main breakage (wasmtime CVE fallout), not caused by this diff. Rebasing on main post-#1041 should make the rollup green. Good to land after that.