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/src/api/rpc/middleware.rs

@@ -21,6 +21,11 @@ pub(super) const UNAUTHENTICATED_METHODS: &[&str] = &[
     "identity.create",
     "identity.verify",
     "identity.resolve-did",
+    // Seed management (onboarding — before user has a session)
+    "seed.generate",
+    "seed.verify",
+    "seed.restore",
+    "seed.save-encrypted",
     // Onboarding restore (before user account exists)
     "backup.restore-identity",
     // Inter-node RPC: called by federated peers over Tor, no session cookies