feat(wallet): pay for peer files from BOTH Cashu and Fedimint ecash (#3)

Paying for a peer file minted a Cashu-only token, so a node whose ecash balance
lived in Fedimint couldn't pay even with funds. Now both backends are tried:

- payer (content.download-peer-paid): mint a Cashu token first; on failure fall
  back to spending Fedimint notes. Only error if BOTH backends can't cover it.
- seller (verify_and_receive_payment): accept Fedimint notes as well as Cashu —
  anything not starting with "cashu" is redeemed via reissue_into_any.
- new fedimint_client::spend_from_any() — spend from whichever joined federation
  has the balance, returning the notes + federation id (mirrors reissue_into_any).
- wallet.ecash-balance now also reports fedimint_sats + combined total_sats; the
  pay-for-file pre-check uses the combined total so a Fedimint-funded node isn't
  wrongly blocked.

Compiles (cargo check + vue-tsc). Live cross-node federation validation pending
(dual-ecash phase 6) — needs two nodes sharing a federation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/content.rs

@@ -383,10 +383,35 @@ impl RpcHandler {
             return Err(anyhow::anyhow!("Invalid v3 onion address"));
         }
 
-        // Mint ecash payment token
-        let token_str = ecash::send_token(&self.config.data_dir, price_sats)
-            .await
-            .context("Failed to create ecash payment token — check wallet balance")?;
+        // Mint an ecash payment token, trying BOTH backends: Cashu first, then
+        // Fedimint. The seller's verify_payment_token accepts either, so a node
+        // whose balance lives in one system can still pay (#3). Surface the
+        // Cashu error only if BOTH paths fail.
+        let token_str = match ecash::send_token(&self.config.data_dir, price_sats).await {
+            Ok(t) => t,
+            Err(cashu_err) => {
+                match crate::wallet::fedimint_client::spend_from_any(
+                    &self.config.data_dir,
+                    price_sats,
+                )
+                .await
+                {
+                    Ok((notes, _fed_id)) => notes,
+                    Err(fedi_err) => {
+                        tracing::warn!(
+                            "paid download: no ecash backend could pay {price_sats} sats \
+                             (cashu: {cashu_err:#}; fedimint: {fedi_err:#})"
+                        );
+                        return Ok(serde_json::json!({
+                            "error": format!(
+                                "Couldn't pay {price_sats} sats from your ecash wallet \
+                                 (Cashu or Fedimint). Fund either wallet and try again."
+                            )
+                        }));
+                    }
+                }
+            }
+        };
 
         let (data, _) = self.state_manager.get_snapshot().await;
         let local_did = crate::identity::did_key_from_pubkey_hex(&data.server_info.pubkey)?;