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/api/rpc-client.ts

@@ -225,6 +225,43 @@ class RPCClient {
     })
   }
 
+  // ─── Seed Management ───────────────────────────────────────────────
+
+  async generateSeed(): Promise<{ words: string[] }> {
+    return this.call({ method: 'seed.generate' })
+  }
+
+  async verifySeed(words: string[], indices: number[]): Promise<{
+    verified: boolean
+    did: string
+    nostr_npub: string
+  }> {
+    return this.call({ method: 'seed.verify', params: { words, indices } })
+  }
+
+  async restoreSeed(words: string[]): Promise<{
+    did: string
+    nostr_npub: string
+    restored: boolean
+  }> {
+    return this.call({ method: 'seed.restore', params: { words } })
+  }
+
+  async saveSeedEncrypted(passphrase: string): Promise<{ saved: boolean }> {
+    return this.call({ method: 'seed.save-encrypted', params: { passphrase } })
+  }
+
+  async seedStatus(): Promise<{
+    has_seed: boolean
+    is_legacy: boolean
+    identity_count: number
+    next_index: number
+  }> {
+    return this.call({ method: 'seed.status' })
+  }
+
+  // ─── Node Identity ───────────────────────────────────────────────
+
   async getNodeDid(): Promise<{ did: string; pubkey: string }> {
     return this.call({
       method: 'node.did',