feat: BIP-39 master seed for unified key derivation

Replace fragmented random key generation with a single 24-word BIP-39
mnemonic that deterministically derives all node keys: Ed25519 (DID),
secp256k1 (Nostr/Bitcoin), BIP-84 xprv (Bitcoin Core), and LND aezeed
entropy. New onboarding flow: seed generate → word verification → identity
naming. Restore path enabled via 24-word entry. Includes seed RPC handlers,
mock backend support, LND/Bitcoin Core wallet-from-seed integration, and
UI polish across settings and discover views.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/Cargo.toml

@@ -54,6 +54,10 @@ hex = "0.4"
 bs58 = "0.5"
 chrono = "0.4"
 
+# BIP-39 mnemonic seed generation + BIP-32 HD key derivation
+bip39 = { version = "=2.1.0", features = ["rand"] }
+bitcoin = { version = "=0.32.5", features = ["rand-std"] }
+
 # Configuration
 toml = "0.8"
 serde_yaml = "0.9"