feat(formplayer): add QrcodeQuestionRenderer for in-form QR code scanning

[najuna-brian]

Description

Adds the missing QrcodeQuestionRenderer so that schema fields with "format": "qrcode" render a native QR scanner control inside forms.

Closes #325

What changed

File	Change
formulus-formplayer/src/renderers/QrcodeQuestionRenderer.tsx	New renderer — scan button, scanned value display, rescan/delete, manual text entry fallback
formulus-formplayer/src/App.tsx	Import + register in customRenderers (priority 10, formatIs('qrcode'))

How it works

• Calls FormulusClient.requestQrcode(fieldId) → stores result.data.value as a plain string
• Shows a tap-to-scan button when empty, a green chip with the value when scanned
• Manual entry toggle for when the scanner is unavailable
• Follows the same pattern as PhotoQuestionRenderer and SignatureQuestionRenderer
• Wrapped in QuestionShell for consistent title/description/error display

Schema usage

{
  "qr_field": {
    "type": "string",
    "format": "qrcode",
    "title": "Scan QR code"
  }
}

Testing

• Dev mock (webview-mock.ts) already simulates requestQrcode — works out of the box in dev mode
• Verified: zero ESLint errors, Prettier formatted