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>

neode-ui/src/views/OnboardingPath.vue

@@ -109,6 +109,6 @@ onMounted(() => {
 
 function proceed() {
   playNavSound('action')
-  router.push('/onboarding/did').catch(() => {})
+  router.push('/onboarding/seed').catch(() => {})
 }
 </script>