Add native handling for bitcoin:, lightning: and lnurl: links

Goal
Make Zap Browser behave like a Lightning-native browser.

Tasks
- Detect bitcoin: links
- Detect lightning: invoices
- Detect LNURL links
- Open native payment UI
- Add confirmation dialogs
- Add optional auto-handler settings

Acceptance criteria
- Clicking payment links opens native browser payment flow
- No manual copy/paste required