FIX: generate LNURL server-side for unique voucher links

[PatMulligan]

Summary

• Fix display page and QR code dialog to use server-generated LNURL URLs
• Add lnurl_url field to API responses for proper unique voucher support
• Restores correct URL generation for unique voucher withdraw links

Fixes #64

Problem

Commit 10a4caf (feat: add lud17 support) introduced a regression where both the display page and the index page QR code dialog construct LNURL URLs client-side using only the unique_hash.

For unique voucher links (is_unique=True), the URL must include both unique_hash AND id_unique_hash in the path:

• ❌ Broken: /api/v1/lnurl/{unique_hash}
• ✅ Correct: /api/v1/lnurl/{unique_hash}/{id_unique_hash}

This caused unique voucher links (created via "Quick vouchers" or with the unique checkbox enabled) to return:

{"status":"ERROR","reason":"This link requires an id_unique_hash."}

Solution

• views.py: Generate LNURL server-side for display page using create_lnurl()
• views_api.py: Add lnurl_url field to all API responses
• models.py: Add lnurl_url field to WithdrawLink model
• index.js: Use lnurl_url from API instead of client-side URL construction
• display.html: Use server-provided lnurl_url

The create_lnurl() helper already handled unique links correctly - the fix ensures all UI code paths use it.

Test plan

• Create a regular (non-unique) withdraw link and verify both the display page and QR dialog show working LNURLs
• Create a unique voucher via "Quick vouchers" and verify the QR code works
• Create a unique voucher via "Advanced" with the unique checkbox and verify the QR code works
• Verify QR codes can be scanned and redeemed in a LNURL-compatible wallet

🤖 Generated with https://claude.ai/claude-code