feat(lightning): instant pay feedback, balances never vanish mid-payment

Framework-pt report: a paid invoice stalled the UI with no success
shown, and lightning/total balances disappeared until it settled.
Three compounding causes, three fixes:

- Backend payinvoice's synchronous wait drops 120s → 8s. Fast payments
  (the majority) still settle in one round trip; slow multi-hop routes
  return pending + payment_hash quickly and the caller's 3s poll takes
  over — instead of the modal freezing for up to two minutes.
- payLightningInvoice gains an onPending hook: SendBitcoinModal and the
  scan modal now flip to a visible "Settling…" success pane the moment
  the payment goes pending (safe to close), and the ongoing poll
  upgrades it to Paid — or replaces it with LND's real failure.
- One slow lnd.getinfo poll (5s budget) flipped the Home wallet card to
  "disconnected", hiding balances the user already knew. Three
  consecutive failures are now required (~30s) before the card gives up;
  last-known balances keep rendering throughout.

rpc-client tests 75/75.

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

neode-ui/src/api/rpc-client.ts

@@ -445,7 +445,7 @@ class RPCClient {
   async payLightningInvoice(params: {
     payment_request: string
     amount_sats?: number
-  }): Promise<{
+  }, onPending?: (payment_hash: string) => void): Promise<{
     status: 'succeeded' | 'failed' | 'pending'
     payment_hash: string
     amount_sats: number
@@ -470,6 +470,11 @@ class RPCClient {
     }
     if (!hash) return { status: 'pending', payment_hash: '', amount_sats: amount }
 
+    // Let the caller unblock its UI right now ("settling…") — the backend
+    // answers pending after ~8s, and freezing a modal through the poll loop
+    // below was the "paid but the interface stalled" report.
+    try { onPending?.(hash) } catch { /* caller UI must not break polling */ }
+
     // Poll to a terminal state: every 3s for up to 2 minutes.
     for (let i = 0; i < 40; i++) {
       await new Promise((r) => setTimeout(r, 3000))