Add trade share cards

[Jiangzhewen]

Summary

• Adds a Share Trade modal that renders a 1200x675 PNG card in the browser with Canvas.
• Adds share actions for active Positions and History rows.
• Supports image download, native share when available, and copy-text fallback.
• Keeps the History tools column available even when columns are customized.

Validation

• npm ci
• npm run build
• git diff --check
• Local page load at http://127.0.0.1:5175/trade/ETH-USD; CAP country gating plus data.cap.io / Alchemy failures are external environment issues and not introduced by this PR.

I requested assignment / consideration on #21 and understand bounty eligibility depends on maintainer confirmation because the issue has historical assignment context.

Closes #21

[Jiangzhewen]

Local validation is passing: npm run build completes successfully. The only failing check is the repository-level Vercel integration returning "Account is blocked", which is the same upstream Vercel account issue currently affecting the other open PRs.