feat: replace clipboard copy with QR code for secret key backup (Closes #348)

[waterWang]

Summary

Implements the SEC-25 security fix: replaces the primary clipboard-copy approach for secret key backup with a QR code display, keeping clipboard as a secondary option with stronger security warnings and rate limiting.

Changes

ExportSecretKeyModal.tsx — New QR code primary backup flow

• QR code display as the primary backup method using qrcode.react (already in dependencies)
• User can screenshot or print the QR code — the secret key never touches the clipboard
• Keep clipboard copy as a secondary option with explicit security warning
• Session-level rate limiting: max 1 clipboard export per 5 minutes
• Auto-clear clipboard: clipboard is cleared 30 seconds after copy (existing behavior preserved)
• Clipboard risk warning banner: shown after copying to inform users about Android clipboard risks
• Masked secret key display (hidden by default) with reveal toggle

i18n updates

• Added 5 new translation keys in both en.json and es.json:
  • exportKeyQR — QR code section label
  • exportKeyQRHint — Hint about screenshotting/printing the QR code
  • exportClipboardWarning — Warning about Android clipboard risks
  • exportRateLimited — Rate limit message
  • exportKeyError — Error state message

CSS

• Added @keyframes fadeIn animation for the clipboard warning banner

Tests

• Added comprehensive test suite covering:
  • QR code rendering after loading
  • Clipboard copy with warning banner
  • Rate limiting after copy
  • Auto-clear clipboard after 30s
  • Secret key reveal/hide toggle
  • Close button behavior
  • Error state when loading fails

Acceptance Criteria

• ✅ QR code display as primary backup method (user screenshots/prints)
• ✅ Clipboard copy retained as secondary with explicit warning
• ✅ Auto-clear clipboard within 30s with visible countdown
• ✅ Rate limiting: max 1 export per 5 minutes per session
• ✅ Explicit warning about Android clipboard risks
• ✅ TypeScript passes (no new errors)
• ✅ Backward compatible — no breaking changes

Closes #348