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/mock-backend.js

@@ -917,6 +917,31 @@ app.post('/rpc/v1', (req, res) => {
         return res.json({ result: { valid: true } })
       }
 
+      // BIP-39 seed management (mock for dev mode)
+      case 'seed.generate': {
+        const mockWords = [
+          'abandon', 'ability', 'able', 'about', 'above', 'absent',
+          'absorb', 'abstract', 'absurd', 'abuse', 'access', 'accident',
+          'account', 'accuse', 'achieve', 'acid', 'acoustic', 'acquire',
+          'across', 'act', 'action', 'actor', 'actress', 'actual'
+        ]
+        return res.json({ result: { words: mockWords } })
+      }
+      case 'seed.verify': {
+        const mockDid = 'did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH'
+        return res.json({ result: { verified: true, did: mockDid, nostr_npub: 'npub1mock...' } })
+      }
+      case 'seed.restore': {
+        const mockDid = 'did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH'
+        return res.json({ result: { did: mockDid, nostr_npub: 'npub1mock...', restored: true } })
+      }
+      case 'seed.save-encrypted': {
+        return res.json({ result: { saved: true } })
+      }
+      case 'seed.status': {
+        return res.json({ result: { has_seed: true, is_legacy: false, identity_count: 1, next_index: 1 } })
+      }
+
       case 'node.createBackup': {
         const { passphrase } = params || {}
         if (!passphrase) {