keys.html: generate a BIP39 mnemonic (two-way BIP39)

Bundle the canonical 2048-word English wordlist (data/bip39-english.txt, 13KB).
wasm-keygen.js: entropyToMnemonic + generateMnemonic (entropy → checksum → 11-bit
words). keys.html Generate now produces a 12-word BIP39 mnemonic (exportable to
any wallet) instead of a raw seed; Import still works. test-bip39.mjs covers both
directions against the BIP39/BIP84 vectors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Melvin Carvalho