Debug-log every API request and response

Add debugFetch wrapper in log.js that logs method, URL, and body on
request, and status code on response. Replace all fetch() calls
across balances, transactions, tokens, background RPC proxy, and
settings validation with debugFetch.

src/shared/log.js

@@ -27,4 +27,14 @@ const log = {
     },
 };
 
-module.exports = { log };
+// Fetch wrapper that debug-logs every request and response.
+async function debugFetch(url, opts) {
+    const method = (opts && opts.method) || "GET";
+    const body = opts && opts.body;
+    log.debugf("fetch →", method, url, body || "");
+    const resp = await fetch(url, opts);
+    log.debugf("fetch ←", resp.status, url);
+    return resp;
+}
+
+module.exports = { log, debugFetch };