fix(wallet): Minibits default Cashu mint, resilient peer-file invoices, named default federation

- Cashu default mint was the local Fedimint guardian (:8175), wrongly surfacing
  a Fedimint URL in the Cashu mints list. Default is now Minibits
  (https://mint.minibits.cash/Bitcoin) — Cashu and Fedimint are distinct
  protocols (Fedimint lives under its own tab).
- Peer-file (buy) invoice creation: retry the LND REST call (3× / 400ms) so a
  transient LND-REST blip (swap pressure / just-restarted / TLS race) no longer
  hard-fails as an opaque 503, and surface the real error chain ({:#}) in the
  response + logs instead of a generic "Failed to create invoice".
- Autojoined default federation now shows a friendly name ("Archipelago
  Federation") in the Fedimint tab instead of a bare federation id.

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

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

@@ -222,8 +222,12 @@ impl ApiHandler {
                 hyper::Body::from(r#"{"error":"Invoice missing payment hash"}"#),
             )),
             Err(e) => {
+                // Surface the FULL error chain ({:#}) — the generic top-level
+                // message hid the real cause (e.g. the LND REST connection
+                // failing), which made this 503 undiagnosable.
+                tracing::warn!("content invoice creation failed: {e:#}");
                 let body = serde_json::json!({
-                    "error": format!("Could not create invoice: {e}")
+                    "error": format!("Could not create invoice: {e:#}")
                 });
                 Ok(build_response(
                     StatusCode::SERVICE_UNAVAILABLE,