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/background-account-history-settings.test.js

@@ -61,6 +61,7 @@ const DEFAULT_HOTMAIL_LOCAL_BASE_URL = 'http://127.0.0.1:17373';
 const DEFAULT_ACCOUNT_RUN_HISTORY_HELPER_BASE_URL = DEFAULT_HOTMAIL_LOCAL_BASE_URL;
 const DEFAULT_HOTMAIL_REMOTE_BASE_URL = '';
 const DEFAULT_VERIFICATION_RESEND_COUNT = 4;
+const DEFAULT_SUB2API_PROXY_NAME = 'shadowrocket';
 const HOTMAIL_SERVICE_MODE_REMOTE = 'remote';
 const HOTMAIL_SERVICE_MODE_LOCAL = 'local';
 const VERIFICATION_RESEND_COUNT_MIN = 0;
@@ -109,4 +110,12 @@ return {
     api.normalizeAccountRunHistoryHelperBaseUrl(''),
     'http://127.0.0.1:17373'
   );
+  assert.equal(
+    api.normalizePersistentSettingValue('sub2apiDefaultProxyName', ''),
+    'shadowrocket'
+  );
+  assert.equal(
+    api.normalizePersistentSettingValue('sub2apiDefaultProxyName', ' proxy-a '),
+    'proxy-a'
+  );
 });