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/system/mod.rs

@@ -156,6 +156,7 @@ pub(super) fn parse_meminfo_kb(val: &str) -> Result<u64> {
 
 /// Read disk usage via `df` for the root filesystem.
 /// Returns (used_bytes, total_bytes).
+#[allow(dead_code)]
 pub(super) async fn read_disk_usage() -> Result<(u64, u64)> {
     read_disk_usage_path("/").await
 }