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/OnboardingWrapper.vue

@@ -75,6 +75,7 @@ const transitionName = ref('depth-forward')
 // Ordered onboarding steps for direction detection
 const onboardingOrder = [
   '/onboarding/intro', '/onboarding/path', '/onboarding/options',
+  '/onboarding/seed', '/onboarding/seed-verify', '/onboarding/seed-restore',
   '/onboarding/did', '/onboarding/identity', '/onboarding/backup',
   '/onboarding/verify', '/onboarding/done', '/login'
 ]
@@ -96,8 +97,11 @@ const routeBackgrounds: Record<string, string> = {
   '/onboarding/intro': 'bg-intro.jpg', // Video will be used instead
   '/onboarding/options': 'bg-intro-4.jpg',
   '/onboarding/path': 'bg-intro-3.jpg',
-  '/onboarding/did': 'bg-intro-5.jpg',
-  '/onboarding/identity': 'bg-intro-5.jpg',
+  '/onboarding/seed': 'bg-intro-5.jpg',
+  '/onboarding/seed-verify': 'bg-intro-6.jpg',
+  '/onboarding/seed-restore': 'bg-intro-2.jpg',
+  '/onboarding/did': 'bg-intro-4.jpg',
+  '/onboarding/identity': 'bg-intro-1.jpg',
   '/onboarding/backup': 'bg-intro-6.jpg',
   '/onboarding/verify': 'bg-intro-2.jpg',
   '/onboarding/done': 'bg-intro-1.jpg',