chore: show QR with lowercase invoice value

[im-adithya]

Describe the changes you have made in this PR

From unes on Telegram:

when generating an invoice using the extension then scan the qr code using a mobile app or a centralized exchange the invoice url is in uppercase and some apps does not allow this

Is there any reason why we use toUpperCase()?

Type of change

(Remove other not matching type)

• fix: Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Manually

Checklist

• [x] Self-review of changed code
• [x] Manual testing
• [x] Added automated tests where applicable
• [x] Update Docs & Guides
• For UI-related changes
• [x] Darkmode
• [x] Responsive layout

[im-adithya]

Sorry for opening this. QRs should always have BOLT-11 in upper case. See requirements.