Dev: proxy all regtest backends through the dev server for LAN/phone testing

The dev server already binds 0.0.0.0, but the app's regtest data source + SP
indexer pointed at localhost, which on a phone means the phone's own localhost.
Proxy them all through the dev server (same-origin), like /boltz already was:
HTTP by prefix (/boltz, /esplora, /sp) and WebSocket by exact path (/electrum →
Fulcrum, /sp/ws → SP indexer), with a Bun websocket relay. The regtest presets
become relative paths, and a small absWsUrl() resolves '/path' WS URLs to
ws(s)://<page-host>/path so they work from any device hitting the dev server.

No per-device config, no CORS. Verified: /electrum relays Fulcrum 2.1.1, /esplora
returns the tip height, and the desktop wallet still loads (balance 98,444) and
connects through the proxy with no errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

2 files changed, 16 insertions, 5 deletions

diff --git a/src/api.js b/src/api.js
index a12ddc6..186a0ca 100644
--- a/src/api.js
+++ b/src/api.js
@@ -76,8 +76,8 @@ const DATA_SOURCES_BY_NET = {
     { id: 'custom', label: 'Custom', type: 'auto', url: '' },
   ],
   regtest: [
-    { id: 'local', label: 'Local Fulcrum', type: 'electrum', url: 'ws://localhost:50003' },
-    { id: 'localesplora', label: 'Local Esplora', type: 'esplora', base: 'http://localhost:3000', web: 'http://localhost:3000', kind: 'esplora' },
+    { id: 'local', label: 'Local Fulcrum', type: 'electrum', url: '/electrum' },
+    { id: 'localesplora', label: 'Local Esplora', type: 'esplora', base: '/esplora', web: 'http://localhost:3000', kind: 'esplora' },
     { id: 'custom', label: 'Custom', type: 'auto', url: '' },
   ],
 };
@@ -185,7 +185,7 @@ const SP_INDEXER_PRESETS_BY_NET = {
   mainnet: [{ id: 'coinos', label: 'coinos', url: 'https://sp.coinos.io' }, { id: 'custom', label: 'Custom', url: '' }],
   testnet: [{ id: 'custom', label: 'Custom', url: '' }],
   mutinynet: [{ id: 'custom', label: 'Custom', url: '' }],
-  regtest: [{ id: 'local', label: 'Local (localhost:8888)', url: 'http://localhost:8888' }, { id: 'custom', label: 'Custom', url: '' }],
+  regtest: [{ id: 'local', label: 'Local (proxied)', url: '/sp' }, { id: 'custom', label: 'Custom', url: '' }],
 };
 const SP_INDEXER_DEFAULT = { mainnet: 'coinos', testnet: 'custom', mutinynet: 'custom', regtest: 'local' };
 const SP_INDEXER_KEY = 'btc-wallet-sp-indexer';
diff --git a/src/wallet.js b/src/wallet.js
index bc70561..519e3e1 100644
--- a/src/wallet.js
+++ b/src/wallet.js
@@ -27,6 +27,17 @@ import { NostrSync, getSyncConfig, encryptGiftPayload, npubOf } from './nostr.js
 import { isSilentPaymentAddress, decodeSilentPaymentAddress, silentPaymentScripts, silentPaymentPlaceholder, deriveSilentPaymentKeys, encodeSilentPaymentAddress, silentPaymentScan, silentPaymentOutputPrivKey, silentPaymentCandidate, bloomHas } from './silentpay.js';
 import { schnorr } from '@noble/curves/secp256k1';
 
+// Resolve a same-origin '/path' WebSocket URL to an absolute ws(s):// URL against
+// the page. The regtest backends are proxied through the dev server as relative
+// paths (e.g. '/electrum') so the app works from a phone over the LAN/Tailscale;
+// absolute URLs (real deployments, tests) pass through untouched.
+function absWsUrl(u) {
+  if (u && u[0] === '/' && typeof location !== 'undefined') {
+    return (location.protocol === 'https:' ? 'wss://' : 'ws://') + location.host + u;
+  }
+  return u;
+}
+
 // No look-ahead: stop scanning a chain at the first unused address. This wallet
 // only ever exposes ONE unused address at a time (freshReceive = first unused;
 // there is no "generate another address" button), so used addresses stay
@@ -528,7 +539,7 @@ export class Wallet {
     this._spDisconnect();
     this._spWsUrl = url;
     let ws;
-    try { ws = new WebSocket(url); } catch { return; }
+    try { ws = new WebSocket(absWsUrl(url)); } catch { return; }
     this._spWs = ws;
     ws.onopen = () => {
       this.spLive = true;
@@ -1772,7 +1783,7 @@ export class Wallet {
     if (getBackend() === 'electrum') {
       if (!this._wsCandidates || !this._wsCandidates.length) { this._wsCandidates = electrumCandidates(this.netName); this._wsCandIdx = 0; }
       const list = this._wsCandidates;
-      return list.length ? list[(this._wsCandIdx || 0) % list.length] : null;
+      return list.length ? absWsUrl(list[(this._wsCandIdx || 0) % list.length]) : null;
     }
     return null;
   }