fix: check AAAA records in DNS rebinding protection (IPv6 bypass)

[Gonzih]

Problem

resolvesToBlockedIp() in browse/src/url-validation.ts only calls resolve4() — it only checks IPv4 A records. An attacker can bypass the DNS rebinding protection by setting up a hostname that has only an AAAA (IPv6) record pointing to a cloud metadata endpoint (or an IPv6 equivalent). Closes #668.

Example bypass: a domain with only AAAA ::ffff:169.254.169.254 (IPv4-mapped IPv6) would sail right through the existing check.

Fix

• Added resolve6() alongside resolve4() — both run concurrently via Promise.allSettled
• Individual failures (e.g. host has no AAAA records) are silently ignored, same as before
• Added ::ffff:169.254.169.254 and fe80::1 to BLOCKED_METADATA_HOSTS to catch IPv4-mapped and link-local IPv6 metadata patterns
• Used Promise.allSettled so a failure on one record type doesn't prevent checking the other

Why Promise.allSettled not Promise.all

Many hosts legitimately have no AAAA records. resolve6() throws ENOTFOUND in those cases. Using allSettled collects both results and lets us check all resolved addresses without a try/catch per record type.

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