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/marketplace.rs

@@ -138,6 +138,16 @@ pub enum ManifestDescription {
     Detailed { short: String, long: String },
 }
 
+impl ManifestDescription {
+    /// Return the short description regardless of variant.
+    #[allow(dead_code)]
+    pub fn short(&self) -> &str {
+        match self {
+            Self::Simple(s) => s,
+            Self::Detailed { short, .. } => short,
+        }
+    }
+}
 
 /// A discovered marketplace app with trust scoring.
 #[derive(Debug, Clone, Serialize, Deserialize)]