Add trade sharing card flow

[BigPolarBear]

Summary

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

Validation

• npm install
• npm run build
• git diff --check
• Local /trade route returned HTTP 200 from sirv build output.

I requested assignment / consideration on #21 and understand bounty eligibility depends on maintainer confirmation because the issue has old assignment history and another active submission. This is submitted as an alternate focused implementation using the current rendered client data.

Payout target if accepted: USDT to 0x38938CB6Fd54172C5E82a5d01a9236fF12E84BdF.

Closes #21