feat(wallet): on-chain send fee control + BTC/sats amount entry

- sats/BTC unit toggle on the on-chain amount field with live conversion
  hint; canonical value stays sats end-to-end
- Fast / Standard / Slow fee presets (1 / 6 / 144 block targets) plus a
  custom pane taking target blocks or an explicit sat/vB rate
- confirm pane shows LND's fee estimate for the chosen speed via the new
  lnd.estimatefee RPC (GET /v1/transactions/fee)
- lnd.sendcoins now accepts target_conf / sat_per_vbyte with the same
  mutual-exclusion + range validation as channel opens

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

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

@@ -129,6 +129,7 @@ impl RpcHandler {
             "lnd.closechannel" => self.handle_lnd_closechannel(params).await,
             "lnd.newaddress" => self.handle_lnd_newaddress().await,
             "lnd.sendcoins" => self.handle_lnd_sendcoins(params).await,
+            "lnd.estimatefee" => self.handle_lnd_estimatefee(params).await,
             "lnd.createinvoice" => self.handle_lnd_createinvoice(params).await,
             "lnd.payinvoice" => self.handle_lnd_payinvoice(params).await,
             "lnd.create-psbt" => self.handle_lnd_create_psbt(params).await,

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

@@ -124,16 +124,41 @@ impl RpcHandler {
             return Err(anyhow::anyhow!("Invalid Bitcoin address format"));
         }
 
+        // Fee control: either a confirmation target or an explicit fee rate
+        let target_conf = params.get("target_conf").and_then(|v| v.as_i64());
+        let sat_per_vbyte = params.get("sat_per_vbyte").and_then(|v| v.as_i64());
+        if target_conf.is_some() && sat_per_vbyte.is_some() {
+            return Err(anyhow::anyhow!(
+                "Invalid fee parameters: specify either target_conf or sat_per_vbyte, not both"
+            ));
+        }
+        if let Some(tc) = target_conf {
+            if !(1..=1008).contains(&tc) {
+                return Err(anyhow::anyhow!(
+                    "Invalid target_conf: must be between 1 and 1008 blocks"
+                ));
+            }
+        }
+        if let Some(rate) = sat_per_vbyte {
+            if !(1..=5000).contains(&rate) {
+                return Err(anyhow::anyhow!(
+                    "Invalid sat_per_vbyte: must be between 1 and 5000"
+                ));
+            }
+        }
+
         info!(
             addr = addr,
             amount = amount,
             send_all = send_all,
+            target_conf = target_conf,
+            sat_per_vbyte = sat_per_vbyte,
             "Sending on-chain Bitcoin"
         );
 
         let (client, macaroon_hex) = self.lnd_client().await?;
 
-        let send_body = match amount {
+        let mut send_body = match amount {
             Some(amount) => serde_json::json!({
                 "addr": addr,
                 "amount": amount.to_string(),
@@ -143,6 +168,13 @@ impl RpcHandler {
                 "send_all": true,
             }),
         };
+        if let Some(tc) = target_conf {
+            send_body["target_conf"] = serde_json::json!(tc);
+        }
+        if let Some(rate) = sat_per_vbyte {
+            // LND REST encodes uint64 as a JSON string
+            send_body["sat_per_vbyte"] = serde_json::json!(rate.to_string());
+        }
 
         let resp = client
             .post(format!("{LND_REST_BASE_URL}/v1/transactions"))
@@ -171,6 +203,82 @@ impl RpcHandler {
         Ok(serde_json::json!({ "txid": txid }))
     }
 
+    /// Estimate the on-chain fee for sending `amount` sats to `addr` at a
+    /// confirmation target. Returns `{ fee_sat, sat_per_vbyte }` so the send
+    /// UI can show the cost of each preset before the user confirms.
+    pub(in crate::api::rpc) async fn handle_lnd_estimatefee(
+        &self,
+        params: Option<serde_json::Value>,
+    ) -> Result<serde_json::Value> {
+        let params = params.unwrap_or_default();
+        let addr = params
+            .get("addr")
+            .and_then(|v| v.as_str())
+            .ok_or_else(|| anyhow::anyhow!("Missing 'addr' parameter"))?;
+        if addr.len() < 14 || addr.len() > 90 || !addr.chars().all(|c| c.is_ascii_alphanumeric()) {
+            return Err(anyhow::anyhow!("Invalid Bitcoin address format"));
+        }
+        let amount = params
+            .get("amount")
+            .and_then(|v| v.as_i64())
+            .ok_or_else(|| anyhow::anyhow!("Missing 'amount' parameter (sats)"))?;
+        if !(546..=21_000_000 * 100_000_000).contains(&amount) {
+            return Err(anyhow::anyhow!("Invalid amount"));
+        }
+        let target_conf = params
+            .get("target_conf")
+            .and_then(|v| v.as_i64())
+            .unwrap_or(6);
+        if !(1..=1008).contains(&target_conf) {
+            return Err(anyhow::anyhow!(
+                "Invalid target_conf: must be between 1 and 1008 blocks"
+            ));
+        }
+
+        let (client, macaroon_hex) = self.lnd_client().await?;
+
+        let resp = client
+            .get(format!("{LND_REST_BASE_URL}/v1/transactions/fee"))
+            .query(&[
+                (format!("AddrToAmount[{addr}]"), amount.to_string()),
+                ("target_conf".to_string(), target_conf.to_string()),
+            ])
+            .header("Grpc-Metadata-macaroon", &macaroon_hex)
+            .send()
+            .await
+            .context("Failed to estimate fee")?;
+
+        let status = resp.status();
+        let body: serde_json::Value = resp
+            .json()
+            .await
+            .context("Failed to parse fee estimate response")?;
+
+        if !status.is_success() {
+            let msg = body
+                .get("message")
+                .and_then(|v| v.as_str())
+                .unwrap_or("Unknown error");
+            return Err(anyhow::anyhow!("Failed to estimate fee: {}", msg));
+        }
+
+        // LND REST encodes int64 as JSON strings
+        let fee_sat = body
+            .get("fee_sat")
+            .and_then(|v| v.as_str())
+            .and_then(|s| s.parse::<i64>().ok())
+            .unwrap_or(0);
+        let sat_per_vbyte = body
+            .get("sat_per_vbyte")
+            .and_then(|v| v.as_str())
+            .and_then(|s| s.parse::<i64>().ok())
+            .unwrap_or(0);
+        Ok(serde_json::json!({
+            "fee_sat": fee_sat,
+            "sat_per_vbyte": sat_per_vbyte,
+        }))
+    }
+
     /// Create a Lightning invoice.
     /// Create a Lightning invoice and return `(bolt11, payment_hash_hex)`.
     ///