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/data_model.rs

@@ -54,6 +54,9 @@ pub struct ServerInfo {
     pub wifi_ssids: Vec<String>,
     #[serde(rename = "zram-enabled")]
     pub zram_enabled: bool,
+    /// True if this node's keys are derived from a BIP-39 seed.
+    #[serde(rename = "seed-backed", default)]
+    pub seed_backed: bool,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -269,6 +272,7 @@ impl DataModel {
                 unread: 0,
                 wifi_ssids: vec![],
                 zram_enabled: false,
+                seed_backed: false,
             },
             package_data: HashMap::new(),
             peer_health: HashMap::new(),