feat: Add QR code generation for stream sharing

[Austinaminu2]

Description

Users should be able to share streams easily, especially in person or via mobile. Adding QR code generation for stream URLs enables:

• Quick sharing at in-person events (hackathons, conferences)
• Easy mobile-to-mobile stream viewing
• Print-friendly stream receipts

Proposed Solution

Add a "Share" button on the stream detail page that generates a QR code containing the stream URL. Use a lightweight library like qrcode.react or qr-code-styling.

Design

On the stream detail page, the share button opens a modal with:

• QR code image (SVG for crisp rendering)
• Stream URL with copy button
• Download QR as PNG button
• Basic stream info below the QR (amount, recipient, status)

Acceptance Criteria

• "Share" button on stream detail page generates a QR code
• QR encodes the full stream URL (/app/stream/{id})
• QR rendered as SVG for crisp display at any size
• "Download QR" button saves a PNG
• "Copy Link" button copies the stream URL to clipboard
• QR modal is accessible (focus trap, keyboard dismissible)