Make SUB2API proxy defaults available across panel flows

Expose a configurable default proxy for SUB2API, resolve the proxy once,
persist the proxy_id in state, and reuse it across auth URL generation,
code exchange, and account creation so one OAuth flow stays on one proxy.

Constraint: Upstream GitHub permission is READ, so publication must use a fork PR
Rejected: Push to origin | current GitHub viewer permission does not allow upstream branch publication
Rejected: Resolve proxy independently in later steps | could drift from the proxy chosen at auth URL generation
Confidence: high
Scope-risk: moderate
Reversibility: clean
Tested: npm test passed 183/183
Not-tested: Manual browser extension flow against a live SUB2API deployment

tests/sidepanel-account-records-manager.test.js

@@ -82,6 +82,7 @@ test('sidepanel html contains account records overlay and manager script', () =>
   assert.match(html, /id="account-records-list"/);
   assert.match(html, /id="account-records-stats"/);
   assert.match(html, /id="btn-clear-account-records"/);
+  assert.match(html, /id="input-sub2api-default-proxy"/);
   assert.notEqual(managerIndex, -1);
   assert.notEqual(sidepanelIndex, -1);
   assert.ok(managerIndex < sidepanelIndex);