fix: add forward headers to LNBits calls for LNURL generation

[TaprootFreak]

Summary

• LNBits generates LNURLs from the incoming HTTP request URL. In the Docker setup on dfxdev, internal calls arrive as http://lnbits:5000/... which fails LNURL validation (requires HTTPS or localhost/onion)
• This breaks user sign-up completely because LNBits crashes when creating the LNURL-pay link
• Adds LIGHTNING_LNBITS_SITE_URL env var (e.g. https://dev.lightning.space) that injects Host and X-Forwarded-Proto headers into all LNBits HTTP calls
• When set, LNBits generates valid external LNURLs like https://dev.lightning.space/lnurlp/api/v1/lnurl/cb/{id}

Changes

• config.ts: add siteUrl config from LIGHTNING_LNBITS_SITE_URL
• lightning-client.ts: add lnBitsForwardHeaders() helper, wire into httpLnBitsConfig()
• .env.example: document new env var
• Includes prettier formatting of lightning-client.ts (was unformatted)

Deployment

After merge, add to Vaultwarden items:

• LDS DEV: LIGHTNING_LNBITS_SITE_URL=https://dev.lightning.space
• LDS PRD: LIGHTNING_LNBITS_SITE_URL=https://lightning.space

Test plan

• Set LIGHTNING_LNBITS_SITE_URL=https://dev.lightning.space in DEV env
• Deploy to dfxdev
• Verify POST /v1/auth/sign-up returns 201 with accessToken
• Verify LNBits logs show no InvalidUrl errors