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/auth.rs

@@ -97,6 +97,7 @@ impl AuthManager {
 
     /// Ensure a default user exists on first boot.
     /// Called once at startup — creates user with default password if none exists.
+    #[allow(dead_code)]
     pub async fn ensure_default_user(&self) -> Result<()> {
         if self.is_setup().await? {
             return Ok(());