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/wallet.rs

@@ -12,8 +12,23 @@ fn is_cashu_token(token: &str) -> bool {
 impl RpcHandler {
     pub(super) async fn handle_wallet_ecash_balance(&self) -> Result<serde_json::Value> {
         let wallet = ecash::load_wallet(&self.config.data_dir).await?;
+        let cashu_sats = wallet.balance();
+        // Spendable Fedimint balance too, so callers (e.g. the pay-for-file
+        // pre-check) see funds available across BOTH backends (#3). Best-effort:
+        // if fmcd isn't installed/joined this is just 0, never an error.
+        let fedimint_sats = match fedimint_client::FedimintClient::from_node(&self.config.data_dir)
+            .await
+        {
+            Ok(client) => client.total_balance_sats().await.unwrap_or(0),
+            Err(_) => 0,
+        };
         Ok(serde_json::json!({
-            "balance_sats": wallet.balance(),
+            // `balance_sats` stays Cashu-only for back-compat; `total_sats` is the
+            // spendable amount across Cashu + Fedimint.
+            "balance_sats": cashu_sats,
+            "cashu_sats": cashu_sats,
+            "fedimint_sats": fedimint_sats,
+            "total_sats": cashu_sats + fedimint_sats,
             "proof_count": wallet.proofs.iter().filter(|p| !p.spent && !p.reserved).count(),
             "mint_url": wallet.mint_url,
         }))