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>

docs/api-reference.md

@@ -162,7 +162,8 @@ All endpoints use JSON-RPC over HTTP POST to `/rpc/v1`.
 | `lnd.openchannel` | `{ pubkey: string, amount: number }` | `{ funding_txid: string }` | Yes |
 | `lnd.closechannel` | `{ channel_point: string }` | `{ closing_txid: string }` | Yes |
 | `lnd.newaddress` | — | `{ address: string }` | Yes |
-| `lnd.sendcoins` | `{ addr: string, amount: number }` | `{ txid: string }` | Yes |
+| `lnd.sendcoins` | `{ addr: string, amount?: number, send_all?: bool, target_conf?: number, sat_per_vbyte?: number }` | `{ txid: string }` | Yes |
+| `lnd.estimatefee` | `{ addr: string, amount: number, target_conf?: number }` | `{ fee_sat: number, sat_per_vbyte: number }` | Yes |
 | `lnd.createinvoice` | `{ amount: number, memo?: string }` | `{ payment_request: string }` | Yes |
 | `lnd.payinvoice` | `{ payment_request: string }` | `{ preimage: string }` | Yes |
 | `lnd.create-psbt` | `{ outputs: object, ... }` | `{ psbt: string }` | Yes |