fix(wallet): surface LND sweep refusals as readable errors

A sweep of 92 unconfirmed/dust sats failed with LND's debug-flavored
"insufficient input to create sweep tx: input_sum=0 BTC, output_sum=
0.00000092 BTC" — and the RPC sanitizer then masked even that behind
"Operation failed. Check server logs." (framework-pt, 2026-08-06).
The sweep mechanics are untouched (balance minus fee, as always) —
this only makes the refusal say WHY in plain language.

- lnd.sendcoins translates the sweep refusal: balance below Bitcoin's
  dust minimum or not yet confirmed, so no transaction can be built
  (LND's original message kept in parens).
- "Failed to send" joins the sanitizer's user-facing allowlist — the
  same lesson as "Insufficient balance"/"Payment failed" before it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/api/rpc/lnd/wallet.rs

@@ -192,6 +192,19 @@ impl RpcHandler {
                 .get("message")
                 .and_then(|v| v.as_str())
                 .unwrap_or("Unknown error");
+            // LND's sweep refusal reads like a debug dump ("insufficient
+            // input to create sweep tx: input_sum=0 BTC, output_sum=…").
+            // input_sum=0 with a tiny output means the wallet's coins are
+            // unconfirmed or below Bitcoin's dust minimum — say that
+            // (framework-pt sweep of 92 sats, 2026-08-06).
+            if msg.contains("insufficient input to create sweep tx") {
+                return Err(anyhow::anyhow!(
+                    "Failed to send: your on-chain balance is too small or still \
+                     unconfirmed to sweep. Bitcoin cannot build a transaction from \
+                     coins below the dust minimum (~546 sats) or from funds that \
+                     have not confirmed yet. (LND: {msg})"
+                ));
+            }
             return Err(anyhow::anyhow!("Failed to send: {}", msg));
         }
 

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

@@ -79,6 +79,11 @@ pub(super) fn sanitize_error_message(msg: &str) -> String {
         // them in the first place (ecash send, 2026-07-22).
         "Insufficient balance",
         "Insufficient funds",
+        // On-chain send/sweep refusals from LND ("Failed to send: your
+        // on-chain balance is too small or still unconfirmed to sweep…").
+        // Masking sent the operator to journalctl again (framework-pt
+        // sweep, 2026-08-06) — same lesson as the two above.
+        "Failed to send",
         // Lightning payment failures carry LND's reason ("invoice expired.
         // Valid until …", "no route", …) — the user can act on every one of
         // them, and masking sent the operator to journalctl (invoice-expired